18 PHYSIOLOGY contrivances, built up partly of inert tissues, partly of active tissues, such as muscle and nerve. In tracing the food and oxygen into the blood and the waste matters out of the blood, in studying the distribution of the blood itself and the means adopted to maintain its even temperature, we come, as before, on problems partly mechanical or chemical and partly molecular. The changes which the food undergoes in the intestine can be, and have been, successfully studied as a series of purely chemical prob lems conditioned by anatomical arrangements, such as the existence of an acid fluid in the stomach, succeeded by alkaline fluids in the intestine, and the like ; but the ques tions concerned in the discharge of the digestive juices into the alimentary canal, in the secretory activity of the digestive glands, raise up protoplasmic molecular inquiries. In the reception or absorption of the digested food we similarly find the purely physical processes of diffusion and the like overridden by the special protoplasmic activities of the constituent cells of the lining of the canal. In the further elaboration of the digested products the action of cells again intervenes, as it similarly does in the, so to speak, inverted action by which waste matters are cast out of the body, though in both cases the results are in part conditioned by mechanical contrivances. The circulation of the blood is carried on by means of an intricate mechanical contrivance, whose working is determined and whose effects are conditioned by molecular changes occur ring in the constituent muscles and other protoplasmic cells ; the work done by the heart, the varying width of the channels, the transit of material through the filmy capillary walls, all these are at once the results of proto plasmic activity and factors in the mechanical problems of the flow of blood. The oxygen passes into and carbonic acid out of the blood, through simple diffusion, by means of the respiratory pump, which is merely a machine whose motive-power is supplied by muscular energy, and both oxygen and carbonic acid are carried along in the blood by simple chemical means ; but the passage of oxygen from the blood into the tissue and of carbonic acid from the tissue into the blood, though in themselves mere diffusion processes, are determined by the molecular activity of the constituent cells of the tissue. Lastly, the blood, however well prepared, however skilfully driven to the tissue by the well-timed activity of the vascular system, even when it has reached the inner network of the tissue-elements, is not as yet the tissue itself. To become the tissue it must undergo molecular changes of the profoundest kind : it must cross the boundary from dead material to living stuff. The ulti mate problems of nutrition are of the molecular kind. All the machinery, however elaborate, is preparatory only, and it is the last step which costs the most. Of the many problems concerned in these several depart ments of physiology the one class which we have spoken of as being mechanical in nature is far too varied to be treated of as a whole. The problems falling under it have but few features in common ; each stands, as it were, on its own bottom, and has to be solved in its own way. The problems of the other class, however those which we have spoken of as being molecular in nature have a certain common likeness ; and it may be worth while to consider, in a brief and general manner, some of their most striking characters. For this purpose we may first of all turn to the changes arsons takin ^ P lace in a secreting cell, for these have of late and y* 1 " 8 been stu died with signal success. They illustrate changes. w ^at may be called the chemical aspects of vital actions, just as the changes in a muscular fibre, on the other hand, seem to present, in their simplest form, the kinetic aspects of the same actions. If we examine a secreting gland, such as a pancreas or a salivary gland, we find that it is composed of a number of similar units, the unit being a secreting cell of approximately spheroidal form, one part of the surface of which borders a canal continuous with the duct of the gland, while another part is bathed in lymph. The process of secretion consists in the cell discharging into the canal a fluid which is of a specific character, inso much as, though it consists partly of water and other sub stances common to it and other fluids of the body, these are present in it in special proportions ; and it also contains substances or a substance found in itself and nowhere else. To enable it to carry on this work the cell receives supplies of material from the lymph in which it is bathed, the lymph in turn being replenished from neighbouring capillary blood-vessels. The secreting cell itself consists of a soft protoplasmic " body," of the nature previously described, in the midst of which lies a " nucleus." The consideration of the actions carried out by the nucleus may, for simplicity s sake, be left on one side for the present ; and we may regard the cell as a mass of protoplasm consisting, as we have seen, of a network of a particular nature, and of other substances of different nature filling up the meshes or interstices of the network. Such a cell may exist under two different conditions. At one time it may be quiescent : although the blood vessels surrounding it are bathing it with lymph, although this lymph has free access to the protoplasm of the cell, no secretion takes place, no fluid whatever passes from the cell into the canal which it borders. At another time, under, for instance, some influence reaching it along the nerve distributed to the gland, although there may be no change in the quantity or quality of the blood passing through the adjacent blood-vessels, a rapid stream of material flows from the protoplasmic cell -body into the canal. How is this secretion brought about 1 If we examine certain cells, such, for instance, as those of the pancreas, we find that during a period of rest suc ceeding one of activity the cell increases in bulk, and further that the increase is not so much an enlargement of the protoplasmic network as an accumulation of material in the meshes of the network ; in fact, there appears to be a relative diminution of the actual protoplasm, indicating, as we shall see, a conversion of the substance of the net work into the material which is lodged in the interstices of the network. This material may, and frequently does, exist in the form of discrete granules, recognizable under the microscope ; and in the pancreas there is a tendency for these granules to be massed together on the side of the cell bordering the lumen of the canal. During activity, while the cell is discharging its secretion into the canal, these granules disappear, so that the protoplasmic network is after prolonged activity left with a very small burden of material in its meshes ; at the same time there also ap pears to be an accompanying absolute increase of growth of the mass of the protoplasm itself. We have further evi dence that the substance which is thus stored up in the meshes of the cell, forming the granules, for instance, just spoken of, is not, as it exists in the cells, the same sub stance as that which occurs in the secretion as its charac teristic constituent. Thus the characteristic constituent of pancreatic juice is a peculiar ferment body called " tryp- sin," and we possess evidence that the granules in the pan creatic cells are not trypsin. But we have also evidence that these granules consist of material which, upon a very slight change, becomes trypsin, of material which is an ante cedent of trypsin, and which has accordingly been called trypsinogen." Thus the cell during rest stores up tryp- sinogen, and the change which characterizes activity is the conversion of trypsinogen into trypsin, and its consequent discharge from the cell. These are facts ascertained by
observation and experiment, viz., that trypsinogen appears