Microscopical Researches/CLASS IV. Fibre-cells, or tissues, which originate from cells that become elongated into bundles of fibres

by Theodor Schwann

CLASS IV.

Fibre-cells, or tissues, which originate from cells that become elongated into bundles of fibres.

Mere fibres are all that can be detected as the elementary components of the tissues of this class when they are examined in the mature animal. But when we investigate the mode in which they are generated, we see that the fibres are formed only as prolongations of cells, which, in most instances, are elongated in two opposite directions, sometimes terminating at once in a fasciculus of fibres, at other times in a single fibre, which afterwards splits into several finer ones. This constitutes the characteristic feature of the class. We are already acquainted with the type of the prolongation of cells into fibres in the pigment-ramifications, osseous corpuscles, &c. The cells next to be considered differ from them in the following particulars: the fibres originating from any one cell generally lie together in a fasciculus, and in these prolongations of the cells, it is principally the wall which is most strongly developed, whilst, in the former instances, the cells though extended into fibres, were chiefly rendered conspicuous by their cavities. This class comprises the Cellular (areolar), Fibrous, and Elastic tissues.

1. Cellular (areolar) Tissue. This tissue is known to. be composed of extremely minute, tough, smooth fibres, having a pale outline, and usually a serpentine course; they may be seen in their natural state in the mesentery without any dissection. Most areolar tissue may be distended by forcing air into it, and then innumerable cellular spaces are seen communicating with each other in it; it is not known whether these are produced artificially, or whether they existed previously. Areolar tissue also frequently contains fat-vesicles, which, accordiiig to Gurlt, are surrounded by a thin and transparent, but not fibrous, pellicle, often have an hexagonal form, and in that respect resemble vegetable tissue. (Gurlt’s Physiologie der Haussäugethiere, p. 19.) In order to become acquainted with the relation which these constituent parts of areolar tissue bear to the elementary cells, we must refer to the formation of the tissue in the foetus.

If we examine some areolar tissue from the neck, or from the bottom of the orbit of a foetal pig measuring three inches and a half in length, we shall find it to be a gelatinous substance, somewhat more consistent than the vitreous humour of the eye, and, in its earliest state, quite as transparent; as development proceeds, however, it becomes more of a whitish colour, and loses its gelatinous quality. When examined with the microscope, small corpuscles of various kinds are seen in greater or less numbers; they are not, however, sufficiently numerous in a foetus of the size specified to form the entire gelatinous substance, but must necessarily be situated in a transparent, structureless,^[1] primordial substance of a gelatinous nature, which we will for the present call cytoblastema. The whiter this substance appears to the unaided eye, the greater is the number of corpuscles contained in it; their quantity, there- fore, is continually increasing during development, while that of the cytoblastema constantly diminishes. As in consequence of its transparency, the cytoblastema cannot be seen, but is only inferred to exist from the circumstance that the corpuscles, which are visible under the microscope, could not, at the period when they are but few, form the entire jelly, and that when moved, it is plainly seen that they are held together by some invisible medium, so it is no longer possible to convince ourselves of its existence, when the corpuscles are very numerous. It is probable, however, that it remains between the fibres of the areolar tissue throughout life. This cytoblastema is present in the greatest quantity, and therefore most distinctly demonstrable in the jelly which lies between the chorion and amnion in the foetus of the pig at a somewhat more advanced period, and where it may be rendered very clearly perceptible on the margin of the preparation by colouring it with iodine. It is quite as evident in the cellular tissue of the young tadpole. An indistinct fibrous appearance is sometimes given to it by drawing it asunder; but a fibrous structure must not be inferred from that fact simply, since all tenacious matter assumes that appearance under similar circumstances. Since the number of the corpuscles in the cytoblastema continually increases as development proceeds, it would appear that the cytoblastema must be regarded as the primary formation, so that we may suppose some of it to be first present, and then the corpuscles originate in it; at the same time, however, new cytoblastema is formed, in which new corpuscles are in like manner generated, whilst the formation of those in the previously-existing cytoblastema proceeds simultaneously.

Three kinds of these corpuscles may be distinguished in the mammalian embryo; one, which is developed at an earlier period than the rest, and is found in all the areolar tissue throughout the foetus, and two others, which are formed subsequently, and, as it would seem, do not occur in the areolar tissue of some parts. We shall, therefore, designate the first (which is the only essential kind) proper corpuscles of areolar tissue, or—in accordance with the signification which will shortly be determined for them—fibre-cells of areolar tissue; the second kind are fat-cells; the third form round cells of areolar tissue, the precise signification of which I have not yet been able to make out.

a. Proper corpuscles of areolar tissue, or fibre-cells of areolar tissue. The areolar tissue is not found in the same stage of development in every part of the same foetus. When some of the tissue that has reached about its middle stage of development is removed from the neck of a pig’s foetus, measuring from four to seven inches in length, and examined with the microscope, a quantity of corpuscles of various forms are observed in it. The majority of them, however, appear as they are represented in pl. III, fig. 6, a, being spindle-shaped or longish corpuscles, which are thickest in the middle and gradually elongated at both extremities into minute fibres. They may therefore be described as consisting of a thicker portion, or body, and fibres, which proceed from it.

The body is either round or slightly compressed upon the sides. The surface is covered with very minute granules. Within the thickest portion of it lies ‘another small corpuscle of a circular or generally oval form, and which again encloses one or two small dark points, and accords entirely with the common cell-nucleus. It is therefore probable, that the entire corpuscle is a cell containing a nucleus. The nuclei have not a similar size in all the cells; there is a much more striking variation, however, in the relative size of the cells and the nuclei. In the largest cells, such as a, fig. 6, the body is almost as thick again as the nucleus, and it may be observed that the nucleus does not lie in the centre, but upon the wall of the cell. In most instances, however, the cells are relatively smaller, scarcely larger indeed than the nucleus; insomuch, that the fibres often appear to proceed immediately from the nucleus, as at 4 in the figure: the cell in that instance encompasses the nucleus quite closely. Cells frequently become separated during the process of preparation for the microscope, and float about singly in the water, with a portion of the fibres issuing from them. By causing them to roll, when so detached, it may be satisfactorily seen that many of them are somewhat flattened laterally, and that the nucleus is attached to the inside of the cell-wall. The larger cells, under such circumstances, appear as though the granulous aspect were produced by the external wall only, therefore by the cell-membrane, the interior being filled with a clear fluid.

The cells pass by a gradual process of acumination into the fibres, it being quite impossible to discern any defined boundary between them. The fibres are pale, minutely granulous like the cells, and frequently give off branches. Their course is usually straight. It is difficult to find out how they terminate; but they are generally lost in a bundle of extremely minute fibres.

The above-described corpuscles, then, are the fibre-cells of areolar tissue in the middle stage of their development, a condition in which they immediately attract attention in the investigation of that tissue in the foetus. We shall in the next place consider the earlier, and then the subsequent stages of their development. In addition to the corpuscles before mentioned, others may be seen in very young areolar tissue, which are not elongated into fibres, but are more or less round. They are granulous and contain a nucleus with nucleoli, and as they present all the stages of transition up to those cells which are prolonged into fibres, we must regard them as being the un- developed fibre-cells. Various forms of them are delineated in pl. III, fig. 6. I will not assert that all round cells in foetal areolar tissue are young fibre-cells; for we shall presently become acquainted with some which are not. It is only after the commencement of the process of acumination that the young fibre-cells can be distinguished from these; in the earliest state, when they are as yet quite round, almost all cells are alike. It is difficult to determine positively whether or not these cells are formed around a previously existing nucleus; probably, however, such is the case, as there are no cells to be seen without nuclei, although there are many nuclei observed without investing cells.

The following, then, are the results of our investigation into the progress of development of areolar tissue, in so far as we have as yet pursued it. In the first place, small round cells are formed (probably around a previously existing nucleus), in the structureless jelly-like cytoblastema of the tissue. The cells, furnished with the characteristic nucleus, become acuminated in two opposite directions, and these acuminations elongate into fibres, that sometimes give off branches, and at length split into fasciculi of extremely minute fibres, which, in the early stage, cannot be distinctly perceived to be insulated. As development proceeds, the splitting of the two principal fibres, issuing from the body of the cell into a bundle of more minute fibres, continually advances nearer towards the cell, so that, at a later period, a fasciculus of fibres issues immediately from the body of the cell (see pl. III, fig. 7.) At a subsequent period, this process of splitting reaches as far as the nucleus, and at length goes quite through the body of the cell, and the nucleus then lies merely upon a fasciculus of fibres. At the same time the fibres in the progress of development are rendered smooth, become distinctly and individually discernible, and assume their waving course; in short, they acquire the appearance of the ordinary fibres of areolar tissue. (See the figure.) As the process of splitting advances from both sides towards the nucleus, the fibres in its neighbourhood are those which are longest united together, and that part of the cell is the last to undergo division. The nucleus remains for a time lying upon the fasciculus of fibres; and when it is at last absorbed, we have a bundle of fibers in the place of the original cell. The figure represents a nucleated cell, which is elongated at the upper end into the characteristic fibres of areolar tissue, each one being individually perceptible; the upper part of the body of this cell has also begun to split into fibres. With regard to the elongation downwards, it is not possible to distinguish whether there are separate fibres yet formed, and collected into a cord, or whether it is still merely a simple prolongation of the cell.

It now becomes a question how the elongation of the cells into fibres, and their division, and at a later period the splitting of the body of the cell also into more minute fibres, can be conceived to take place. We have already observed a prolongation of the cells into fibres in several instances, and have traced it minutely in the stellated pigment-cells. The only difference between them and the fibre-cells of areolar tissue is, that in the latter, the elongation generally takes place in two opposite directions only, a circumstance which also frequently occurs with pigment-cells; whilst, on the other hand, the cells of areolar tissue also frequently become elongated into fibres on several sides; see, for example, pl. III, fig. 8. There is often a striking resemblance in form between some of the cells of areolar tissue and those of pigment; compare, for instance, pl. III, fig. 6 a, with pl. I, fig. 8 e. Analogy would lead us to regard those fibres as hollow; but since the cell-contents are not so characteristic in them as they are in the pigment-cells, a cavity might really exist, but not fall under observation, in consequence of the minuteness of the fibre; the appearance of the fibres, therefore, proves nothing, either in favour of or against their hollowness. Since, however, we are already acquainted-with many extremely minute hollow prolongations of cells, and as the transformation of the cells into fibres in the areolar tissue, takes place by a gradual acumination, it seems to me, for the present, more probable that they are hollow rather than solid. If we imagine the formation of the fibres from a cell to take place by the cell-wall growing more vigorously at two opposite limited spots than it does at any other part, we can then conceive that the division of these main fibres into branches, and their prolongation into fibrils, may be effected by the same process. The question as to the hollowness or solidity of these fibrils, is still less capable of being settled by observation than that with respect to the larger fibres. Analogy is in favour of their being hollow, and the minuteness of an object forms no limit to nature’s operations.

The splitting into fibres, which, as we have seen, pursues retrograde course from the branches towards the main fibres, and thence towards the body of the cell, might be illustrated in the followimg manner: — suppose that part of a glove which corresponds to the hand to be the body of a cell, and the fingers to be a fasciculus of fibrils. If the membrane situate in the angle between two fingers grow in the direction of the hand, the glove will at length be split into five portions. But a difficulty arises with respect to the fibre-cells of areolar tissue, which is, that the division into fibres advances from two opposite sides towards the body of the cell, and, therefore, the fibres of one side must ultimately correspond with those of the other. This, however, admits of no further explanation than the healing of the corresponding primitive fibres in the reproduction of nerves does. Meanwhile the above are only attempts to convey a clear idea of the results of my investigations, modes of representation which are susceptible of various modifications, provided they be not made to contradict the observations; the latter may be briefly summed up as follows: — cells, furnished with the characteristic nucleus, are present in the first instance, which become elongated on two opposite sides, more rarely on several sides, into fibres, and these are prolonged into more minute fibres. At a later period the principal fibres, and then also the bodies of the cells are split into fibres, so that a small fasciculus of fibrils, with a nucleus fixed upon it, remains in the place of the original single — cell. Last of all, the nucleus also disappears, and fibrils alone remain. All these transformations proceed in a homogeneous cytoblastema, which probably also continues to exist between the fibres of areolar tissue in the adult.

b. Adipose cells. In the later periods of foetal existence, adipose cells occur in many situations in addition to the fibre-cells before described. They are usually first seen in small groups between the fibre-cells. They are round cells of very various sizes, which are generally completely filled by a single fat-globule. The cell-membrane which closely encompasses the contents, is most minutely granulous, or, according to Gurlt, homogeneous. It is in most instances very thin, being about half the thickness of a blood-corpuscle, but sometimes it is much thicker, and in the subcutaneous areolar tissue of the thigh of a rickety child, at the age of twelve months (probably in connexion with the disease), was almost as thick as the breadth of a human blood-corpuscle. In the early stage, this cell-membrane encloses a very distinct nucleus of a round or oval form, which is sometimes flattened. When the former is thin, the nucleus presents itself externally as a little prominence upon the round fat-globule, which is closely encompassed by the cell-membrane ; but when thick, the nucleus lies embedded in it. It contains one or two nucleoli. It is not uncommon for adipose cells to contain a number of small globules instead of one fat-globule, in such instances, one of them is generally remarkable for its larger size. The adipose cells are best seen in the fat found in the cranial cavity of a young carp, before it has attained the length of six inches. (See pl. III. fig. 10.) They there lie in so soft a substance, that they may be insulated without any difficulty, and float singly in the water in which they are examined. Some are so large as to be visible even with the unaided eye. When examined under the microscope with a magnifying power of 450, the cell-membrane is readily recognized, it is very thin, and closely encompasses the contents. It rises into a little prominence on one side, within lies a proportionately large, and very beautiful cell-nucleus, which is oval, but not flattened, and contains one or two very distinct nucleoli. Some of these fat-cells have two such nuclei, which have precisely similar relations to the cell, and both elevate the cell-membrane into a prominence at the points where they are attached. When one of these cells is pressed under the compressorium, the cell-membrane is at first remarkably expanded, and then tears to a very limited extent, allowing the fat to flow out. When the pressure is discontinued, it contracts again strongly. It has a minutely granulous aspect, is soft and very elastic, but not fibrous.

In close apposition, the cells become flattened against one another into polyhedral shapes, and, as Gurlt remarks, they then resemble vegetable cells in their appearance. We, however, may go further, and regard them as corresponding in signification also. In them the fat forms the cell-contents, as the pigment does in its cells, and the ethereal oil, &c. in those of plants. In its physiological signification of nutritive deposit it has more analogy with starch than with any other substance. I know not whether the nucleus is the part first formed in these cells, or not. Nuclei without any investing cells are found in the cranial cavity of the carp, lying with the adipose cells in the surrounding cytoblastema; these, however, may be nuclei of fibre-cells of areolar tissue. Sooner or later the nuclei become absorbed. They were still quite distinct in the adipose cells of the subcutaneous areolar tissue in the thigh of the before-mentioned rickety child twelve months old, whilst I could not detect any in the neck of a foetus at the seventh month. The absorption of the nucleus proceeds in one of two ways; either its external contour becomes gradually indistinct, some granulous substance merely being left in its place, which substance also disappears at a later period, or small fat-globules are formed both within the nucleus itself, and in its immediate proximity, which go on increasing in size, whilst the nucleus gradually disappears. The cell-membrane probably remains, even in the mature condition of the tissue, and Gurlt has made the very interesting observation, that in emaciated persons, the ordinary adipose cells are filled with serum.

c. The third kind of cells which occur in the areolar tissue of the foetus are round, for the most part extremely pale and transparent (pl. III, fig. 9.) They vary very much in size,. most of them being much larger than the fibre-cells, and some as large as the largest adipose cells. They can very rarely be seen without the aid of the most favorable light, but when, under such circumstances, the observer has once detected one of them, and become familiar with the degree of its transparency, they may be recognized in great numbers. They have a distinct nucleus attached to the internal surface of their wall, containing one or two nucleoli. The nucleus always attracts attention first; the cell surrounding it is either quite transparent, and void of granules, or has granulous contents, and this granulous deposit is first formed in the neighbourhood of the nucleus, the remaining portion of the contents being still transparent. (See the figure.) Gradually, the entire contents appear to become granulous. These cells are distinguished from the fibre-cells of areolar tissue by the circumstance of their becoming much larger than the latter, and their not being elongated into fibres, and from the adipose cells, in that they do not contain fat. I have found them in areolar tissue taken from the bottom of the orbit, and from the neck of a feetal pig, but do not know whether they occur in the areolar tissue of all parts of the body; nor can I determine their signification. They might be regarded as cellular spaces which had been produced by the distension of the areolar tissue with air. In such case, they must communicate with one another in the course of their further development. But this appears to me to be somewhat improbable ; and those spaces may be merely artificial productions. I should rather regard the cells in question as a modification of the adipose cells. For since, according to Gurlt, the ordinary adipose cells in the adult may contain mere watery fluid, one may also conceive the cells destined to the formation of fat becoming completely developed, without that formation actually taking place within them. There are, indeed, adipose cells which contain fat even in the earliest stage of their development, but that is no reason why the formation should not take place at a much later period in other cells. The granulous deposit in many of them might be regarded as the transitional step to the formation of fat. The cellular tissue of the foetus differs in its chemical con- stitution from that of the adult, since we cannot obtain any gelatine from it by boiling, none at least which has the property of gelatinizing. The integument was removed from a pig’s foetus measuring four inches in length, cut up into pieces, and steeped in distilled water for a day. It was then boiled for twenty-four hours. The last process caused it to crumble into small particles that clouded the fluid, in which also some large lamellae of epidermis floated. When examined with the microscope the epidermis exhibited the same structure as it did previous to being boiled; the nuclei in the separate cells were also distinct. A quantity of fibre-cells floated in the fluid in the same state as when they, in their recent condition, composed the entire cutis, that is to say, longish corpuscles extended at both extremities into somewhat long fibres. The cell-nucleus could still be distinctly recognized in some of them. Thus the process of boiling, which had not produced any effect upon the fibre-cells or the fibres issuing from them, had dissolved the connecting cytoblastema, by which they had been held together in the recent state. The fluid was then filtered. Acetic acid caused a precipitate which was not dispersed by an excess of acid. A solution of alum produced a much more copious precipitate, which, in like manner, was not redissolved by an excess of alum, or at least not completely. Tincture of gall-nuts caused a thick clouding, spirits of wine only a slight-one. Hydrochloric acid clouded the fluid, and an excess of acid did not render it clear again. These reactions accord with what Güterbock has called pyine, save that the clouding produced in the latter by hydrochloric acid, was redissolved by an excess of the acid. <A portion of the filtered fluid was evaporated almost to dryness, but even after twenty-four hours, there was no trace of the formation of a jelly observable. In order to separate the component particles of this, in all probability, still very heterogeneous fluid, in some degree from one another, some pure alcohol was added to that portion of it which had been so long evaporated, whereby a very copious precipitate was produced. This was separated by filtration and washing, first with pure alcohol, and afterwards with spirits of wine of 80 per cent. strength, then dried, and again dissolved in boiling water. Acetic acid and alum caused precipitates in this solution, which were not again dissolved by the addition of either of those substances in excess. With respect to hydrochloric acid, the result was the same as before described.

It cannot appear at all surprising that the areolar tissue of the foetus differs from that of the adult, when it is known that many cell-membranes undergo a change in their chemical constitution at different stages of their development, and that the growth of the cells is not a mere mechanical expansion.

Previous to quitting the subject of areolar tissue, we must consider some other processes, by means of which a new for- mation of it takes place in the adult. If (as I have already laid down as an axiom in my first essays, Froriep’s Notizen, 1838, Nos. 91, 103, and 112) the formation of cells be really the principle of development of all organic structures, it must apply no less to pathological than to physiological processes; and that it really does so, is proved by the investigations of Henle with reference to the new products resulting from inflammation, namely, exudation, suppuration, and granulation; the results of his observations are communicated in Hufeland’s Journal, vol. lxxxvi, No. 5.

Vogel pronounced the pus-corpuscles to be epithelium, in consequence of their resemblance to epithelial cells, and there was much of probability in the statement, so long as it ap- peared that every diversity in the physiological signification of an elementary structure was based upon a recognizable diversity of formation. But this conclusion lost its importance, when I brought forward the formation of cells as the common principle of development of elementary structures, which were perfectly distinct in a physiological sense, and at the same time showed the most opposite tissues to be developed from cells, which, in the first instance, perfectly resemble each other, and present no distinction either in appearance or in the signification of their individual parts. Henle, however, proved a positive difference between the epithelial cells and pus-corpuscles, for he found that the nuclei of the youngest epithelial cells were not broken down by the action of acetic acid like those of the pus-corpuscles. The latter must, therefore, be regarded as peculiar cells, which are developed in the serum | of pus in the same manner that all other cells originate in their cytoblastema; the only difference being that in this case the cytoblastema is fluid. Beneath the pus, in healing wounds, lie the granulations, composed of a firm cytoblastema, in which lie a quantity of cells. Henle thus describes the microscopical structure of granulations: “The most superficial part presents cells, which resemble the pus-granules, except that their nuclei are not broken down by acetic acid. In the deeper strata, the nuclei are very distinct, and the envelopes are polygonal, in consequence of mutual pressure. Wood has already drawn attention to their resemblance to epithelial cells. Deeper still the envelopes of the cells are found passing through all the gradual transitions of the fibres of areolar tissue, just as in the immature areolar tissue of the embryo. The first rudiments of these fibres are the longish nucleated corpuscles, which Güterbock observed, and compared to the cylindrical epithelium. Hence it follows, that the formation of new cells proceeds upon the surface of the granulations, and that the transformation of the latter into cellular tissue (cicatrix-material, narbensubstanz) proceeds successively from the bottom of the wound towards the surface.” As no gelatine can be obtained from the granulations by boiling, Güterbock thought that those fibres in the granulations and exudations which resemble the areolar tissue ought not to be regarded as the actual fibres of that tissue, but as merely those of fibrin. But, as we have seen above, the entire areolar tissue of the foetus also does not afford any gelatinizing gelatine; and since Henle observed a similar course of development in these fibres to that which I had pointed out in the areolar tissue of the foetus, we must regard them as the young fibres of that tissue (although they may differ from the mature tissue in their chemical qualities), and the granulations as nothing more than a primitive formation of areolar tissue.

A formation of areolar tissue similar to that in the foetus takes place also in exudations resulting from inflammation. R. Froriep (Klin. Kupfertafeln, llte Lief. Weimar, 1837, Th. lxi) had already observed that irregular granules, some of which seemed to be extended on one or both sides into thin fibres, existed in the exudation of pericarditis, in addition to the fibres resembling areolar tissue. ‘These elongated gra- nules of fibrin,” he continues, ‘* seem to be the commencements of the formation of the new mass of tissue, that is, the rudiments of the newly-forming cylindrical fibres of the areolar tissue composing the proper false membranes, or substance of the cicatrix.” Thus Froriep had already observed the gene- ration of fibres, resembling those of areolar tissue, by the elon- gation of corpuscles; what he here calls fibrine globules, are, no doubt, the nucleated fibre-cells becoming elongated into fibres. Henle examined the exudation by which wounds that heal by the first intention are closed, and found, that, in this also, cells are formed which undergo transformation into fibres of areolar tissue by an elongation of their envelope, just as in the foetus. He thence concludes, that the formation of exudations and granulations are essentially similar processes. The exudation-globules (exsudatkugeln) discovered by Valentin, and described also by Gluge, which, according to the former, occur in every form of exudation, are, he says, allied to pus- corpuscles; and Henle also found that their nuclei are like- wise broken down by the action of acetic acid.

Suppuration, therefore, differs from exudation and granulation only in this circumstance, that a more fluid cytoblastema is formed, in which fewer perfect cells are developed. It represents an intermediate stage between the formation of the firm tissues and the true function of secretion; between which two processes again no essential difference exists.

2. Fibrous Tissue. The fibres of tendons and those of areolar tissue, differing but little from each other, and it being impossible to define precisely the respective limits of the two structures in the perfectly developed condition, we accordingly find that they agree in their mode of origin. Cells, resembling the fibre-cells of areolar tissue, are found in the tendons of the foetus at a very early age. They are arranged with their long axis corresponding to that of the tendon, and are prolonged in two opposite directions into fibres, which again subdivide into more minute ones. (See plate III, fig. 11.) These cells split into fibres precisely in the same way as those of areolar tissue; they have a nucleus similar to theirs in shape, which remains for a period, but is at last absorbed, leaving nothing but the fasciculus of fibres persistent. All these processes, however, take place much earlier in fibrous tissue than in the areolar, so that, unless the tissue is investigated in a very young foetus, we can only detect cell-nuclei intermixed with fibres, or nuclei, in whose immediate proximity a small fasciculus of fibres arises on both sides. At an early stage of development the tendons have a gray appearance, not having assumed the white colour of the adult tissue. This fact is probably connected with a chemical difference existing between the young and perfectly developed fibrous tissue, as in areolar tissue. The quantity of the cytoblastema in which these cells are formed, and by which the fibres and tendons, when perfected, are probably connected together, must be extremely small, and cannot in any way be demonstrated by observation. Its existence can only be inferred by analogy with areolar tissue: it will be remembered that it was proved to be present in the foetal condition of that tissue. The quantity of cytoblastema, in comparison to the fibres present, seems to me to be the principal distinction between areolar and fibrous tissue in the adult. The fibrous tissue contains a great many more fibres within a given space than the areolar does, and they are not more minute than those of the latter tissue. There is just as great a difference, however, between fibres of areolar tissue taken from different parts of the body, as there is between the ordinary fibres of tendons and the most common form of areolar tissue, so that a very gradual transition takes place.

3. Elastic Tissue. The distinction between elastic and fibrous tissue is exhibited at a very early period. But my investigations into the history of the development of this tissue are very incomplete, and extended only so far as to render it probable that it presented no exception to the principle of development from cells. I made use of the aorta of a foetal pig and the ligamentum nuchae of a foetal sheep for the purpose. The tissue taken from these two parts was very different in its general character. In a pig’s embryo, of six inches in length, the aorta had already acquired its yellowish colour and perfect elasticity. The external coat could be easily drawn off in long pieces, almost, indeed, as a distinct tube. Having drawn off a small portion of the middle coat (which, in order to avoid any suspicion of epithelium being mixed with it, was so carefully done, that the internal surface of the vessel remained uninjured), and torn it asunder a little, it was examined with the microscope; the first appearance presented was that of a great quantity of isolated cells, floating about in the surrounding fluid, each of which had its peculiar nucleus. (See plate III, fig. 12.) This easy separation of the cells is never seen in the same degree in the areolar and fibrous tissues, as they are there connected together by the cytoblastema, and by the tough fibres issuing from the cells. These cells of the coat of the aorta vary very much in shape. (See the figure.) Some are round, but most of them oblong, some terminate with a blunt extremity, others are acuminated on two or more sides, others again are prolonged into small processes, which again subdivide, but never extend to any great length. Many of them are somewhat compressed laterally. They all have a granulous aspect, but that appearance, so far as one can judge by rolling the cells about, seems to be referrible to the cell-membrane, and the contents appear to be transparent. The nucleus, enclosing one or two nucleoli, is attached to the interior of their walls. It is sometimes round, at others more or less elongated. These cells have become disengaged from the small portion of the coat of the artery before described. When the preparation itself is examined, many more cells are observed in it, and in addition to them, distinct elastic tissue, consisting of a network of minute, elastic, rough (?) (rauher) fibres, such as are found nearest to the internal coat of the aorta in the adult. (See Eulenburg, de Tela elastica, fig. 9.) It does not, however, present any fibres so thick as those which are found in the external layers of the same part. A blighted nucleus may be recognized here and there in the network. What relation, then, do these cells bear to this still delicate, but so far as regards its characteristic features, perfectly-formed elastic tissue? Analogy would lead us to suppose them to be the primitive formation ; I sometimes also thought, that in rare instances I could observe an immediate transition; that I could see, for instance, one of these cells, furnished with a nucleus, pass continuously on one side into a small portion of reticular tissue, resembling in appearance the undoubted elastic tissue, whilst on its other side it retained its perfect cellular figure. But this occurred so rarely, that I am not enabled to state that a transition of these cells into elastic tissue was proved by observation. If, however, such be really the process of formation, as, from analogy, we are entitled to suppose, the bodies of these cells must then take a much more important share in the formation of the fibres than those of areolar tissue do, and the formation of the elastic fibres of the aorta holds a middle position between the generation of the horny fibres in the cortical substance of feathers (see p. 86, and pl. II, fig. 13) and the production of fibres in areolar and fibrous tissues. The reticular appearance of elastic tissue loses its singularity, when it is conceived to be generated in the same manner as those horny fibres in the feather, that is, partly by an elongation of the cells, and partly by a splitting of their bodies. The splitting of the elastic fibres is not to be regarded as an isolated phenomenon, since such division undoubtedly occurs in transitional stages in the development of all forms of areolar and fibrous tissue in the foetus. In this respect the elastic tissue seems to remain at a lower stage of development. Purkinje and Raüschel observed a darkish point in the centre of a transverse section of the elastic fibres of the aorta, and a dotted line in the course of the fibres, and thence inferred the existence of a rudimentary canal in their interior. This supposition, which I must confess formerly struck me as being a very bold one, has much more weight now, inasmuch as it is not improbable that all fibres which are formed by the prolongations of cells (even those of areolar tissue) are hollow, at least, that they are not composed throughout of one uniformly solid mass. If, as an observation of Valentin’s seems to indicate, still more minute fibres may be rendered visible by the aid of caustic potash in those of ordinary elastic tissue, I should be inclined to regard them as analogous to the primitive muscular fibres, whose signification, as we shall subsequently see, differs entirely, in a morphological view, from the primitive fibres of areolar tissue.

Whilst the elastic tissue of the aorta taken from a very young foetal pig exhibited in the manner before described the main characteristics of the tissue, namely, its yellowish colour and elasticity, the ligamentum nuchae of a sheep’s foetus, at a much later period of gestation, was but very slightly developed. It had a gray and translucent appearance, exhibited no elasticity, and when examined with the microscope, presented no trace of its future structure. A gray cord, indistinctly marked with longitudinal fibres, was seen, in which a great many cell-nuclei might be recognized. I did not prosecute any further researches, as the presence of the nuclei was sufficient proof that there was nothing essentially different in the type of its formation.

On casting a retrospective glance over the class of fibre-cells which we have just been considering, we find that it forms a very natural and somewhat strictly defined group amongst the tissues. The tissues comprised in it are generated from nucleated cells, which are transformed into fasciculi of fibres by elongation, in the first place, and by the splitting of the bodies of the cells themselves into separate fibres at a sub- sequent period. The fundamental phenomenon previously described at page 39 is distinctly presented in the formation of these cells; a structureless, gelatiniform mass, the cytoblastema, is first present, and lies outside the cells already formed. The cells are developed in this, the nucleus being, in all pro- bability, the earliest formation. The growth of the cells proceeds, and they become transformed into fibres in the manner described. The quantity of the cytoblastema con- tinually diminishes in proportion to the cells or fibres which are forming, but probably part of it remains persistent be- tween the fibres throughout the whole of life; m the mature condition, however, it exists in greater quantity in areolar than in fibrous or elastic tissue.

The mode of generation teaches us which parts of these tissues correspond to the constituents of those hitherto treated of. The elementary cells of areolar tissue, before undergoing change, correspond morphologically with the cartilage and epithelium-cells, the mucus-corpuscles, &c. ; and as a fasciculus of fibres is generated from each cell of areolar tissue, a whole fasciculus of fibres of areolar tissue accordingly corresponds to what was an individual cartilage- or epithelium-cell, in the pre- vious classes. The structureless cytoblastema between the fibres of areolar tissue corresponds, however, to the firm inter- cellular substance, forming the principal mass of most cartilages, or to the minimum of cytoblastema between the epithelium-cells ; or, lastly, to the fluid, in which the cells of the first class are formed. In this way one can also readily understand how fibro-cartilage forms a gradual transition between true cartilage and fibrous tissue; it only requires that the cartilage- cells pass through the same transformations as the elementary cells of areolar tissue.

As the present class was based upon the alteration in form which the cells comprised in it undergo, it necessarily could not present many modifications in the shape of the cells, and accordingly exhibits throughout merely an elongation of nucleated cells into fasciculi of fibres, and a subsequent splitting of the bodies into fibres. We have already seen the types of these changes in the second class, where the pigment-cells and those of the crystalline lens, &c., became elongated by a more vigorous growth of the cell-membrane at different spots ; and the class now before us merely affords us an instance of the same pro- cess in a higher degree, since here, one side of one of these more highly developed fibre-cells gives origin to a great number, or even a whole fasciculus, of fibres. The cells of the cortex of the feather also furnished us with an example in the same class of the division of the body of the cells into fibres. Inasmuch as the prolongations of the pigment-cells remain hollow, however minutely they may ramify, one may suppose the same to be the case with regard to the fibres of the tissues now under consideration. The decision of this point would, as we shall subsequently see, be of great importance for the theory of nutrition; but it is quite impossible to determine it by observation, in consequence of the cells of this class not possessing any characteristic contents like those of pigment. An observation by Purkinje and Raüschel was quoted, however, in favour of these cells being hollow. If the hollowness of the fibres of areolar tissue, &c., could be proved, there would then be a division of a single cell into many cells at each transformation of a fibre-cell, and thus the fibrous tissues would not lose their cellular character.

The fibre-cells undergo chemical changes during their growth and gradual transformation into the fibres of areolar tissue, since that tissue, when boiled, even long after the forma- tion of fibres has commenced, yields no gelatinizing gelatine. The formation of the fibres of areolar tissue from cells, having been typified already in the second class, it follows that organization, or the presence of blood-vessels, does not establish any essential difference in the growth of the elementary particles; for this class belongs to the perfectly organized tissues, and areolar tissue is highly vascular. The unorganized tissues were formerly said to grow by apposition, and the organized by intussusception. We have already discussed this distinction at page 95. It is so far correct, that the young cells of unorganized tissues are not formed throughout the entire thickness of the tissue, but only in the neighbourhood of that surface, on which they are in contact with vascular substance, and where they therefore obtain the freshest cytoblastema. But if this distinction between the surface and parenchyma of the tissue be not present, in consequence of the blood-vessels being distributed throughout its whole thickness, the young cells are then also generated in every part of the tissue; and such is the case with areolar tissue. The primary distinction, therefore, merely consists in the absence or presence of vessels, the difference in the place of formation of the new cells being but a secondary distinction. The elementary particles grow in both instances and by the same powers. We shall see hereafter how far the presence of vessels facilitates certain processes which occur during growth. The essential phenomena of growth, and, therefore, also the fundamental powers called into activity by it, are similar in both. But why a formation of vessels should take place in areolar tissue and not in epithelium, is a question for future discussion.

1. Vide note at page 39,