Popular Science Monthly/Volume 17/July 1880/Changes of the Circulation During Cerebral Activity
By CHARLES SEDGWICK MINOT, S. B., S. D.
DR. ANGELO MOSSO, the distinguished Professor of Physiology at the University of Turin, has made some observations on the physiology of the brain which, for novelty and interest, have been equaled by but few recent scientific discoveries; for his researches lie in the debatable land between physiology and psychology, and have contributed to the advance of both sciences.
The history of these discoveries illustrates in a very striking manner the unexpected developments to which scientific research sometimes leads. In 1873-'74 Dr. Mosso experimented in the laboratory of Professor Ludwig, at Leipsic, upon the circulation of the blood through the kidney. He took advantage of the fact that death is a double process: 1. The death of the animal as a whole, or that which we ordinarily call death; and, 2. The subsequent and gradual death of the tissues, to which is due the curious phenomenon that an organ survives the animal of which it has formed a part. The most remarkable and familiar instance of this is the heart of the frog or turtle, which may continue to beat, for several days even, after the animal to which it belonged has been killed. So also the vitality of the kidney may be preserved by proper precautions for a considerable period. The organ overlives, as it is expressed in German. To physiologists this overliving of tissues is of the utmost value, because it enables them to perform numerous experiments that would otherwise be difficult or impossible, which latter alternative would have been the case with Mosso's researches upon the renal circulation.
Mosso's experiments were briefly as follows: Their connection with the subject of this article will appear directly. The kidney was taken from a dog immediately after his death, and glass tubes inserted into the artery and the vein, so that blood could be passed through the former into the kidney, under any desired pressure, sufficient to force it through the vessels of the organ into the vein, whence it could pass out by the glass tube, and be collected in a receptacle. It is unnecessary to describe the details of the experiments—the ingenious devices adopted to prevent the drying up of the kidneys, or irregularities of the pressure of the blood; in short, to eliminate those conditions which might disturb the accuracy of the results. Let it suffice to say that the kidney after its removal from the body was found to preserve its vitality to a fuller extent than had been previously supposed. The especial result of Mosso's experiments was the demonstration of the alterations of the circulation which take place in a single organ independently of the central nervous system, from which the kidney was of course entirely severed. It was noted among other things that the capacity of the blood-vessels altered: under one set of circumstances the kidney held more, under another less blood. This could be detected in two ways: first, when the capacity increased, more blood would enter the artery than flowed out from the vein, or, if the capacity diminished, more would flow out than in; and, second, by changes in the volume of the kidney. The latter method was the most valuable, because the changes in the volume could be actually measured. This was accomplished by placing the kidney in a glass case filled with oil. The tubes for the artery and the vein passed out through the side of the case. Through a third hole passed a horizontal glass tube. Now, when the kidney increased in volume it drove the oil into the horizontal glass tube, when it contracted the oil was sucked back again—hence by measuring the distance the oil advanced or retreated, the corresponding increase or diminution of size of the kidney could be exactly determined.
The physiological importance of the changes of the volume of a single organ was thus clearly established. If it was possible to record those changes in an isolated organ, why not try to record them of a part, a limb still connected with the body? The possibility conceived was soon accomplished. The apparatus was altered and improved. In its new form it could be used to record the changes in the volume of the human forearm by tracing them upon a piece of paper in the shape of a series of curves.
The construction of the apparatus, as finally adopted by Mosso, is shown in the accompanying woodcut, copied from the original memoir of its inventor, who calls his instrument a plethysmograph (pletismografo), because it writes the filling (plethitsmo) and unfilling of an object.
The apparatus consists of—
1. A glass cylinder, A B, forty-eight centimetres long and eight or ten in diameter, open at one end, and terminating in a small neck at the other. It is furnished with two openings, C and D, which serve to fill it with water and to introduce a thermometer. These openings may be closed with corks. The arm of the person to be experimented upon is introduced into this cylinder.
2. A broad ring at A of vulcanized rubber to fit around the end of the cylinder and around the arm introduced into it, so as to form a water-tight joint.
3. A board, E, suspended from above, and serving to support the glass vessel, with the included arm.
4. The recording instruments. These have a difficult office to perform, for they must register the changes in the volume of the limb. The forearm is placed in the cylinder A B, which is then filled with water; when the arm swells, the water is driven out through the rubber tube F; when the arm shrinks, the water is sucked back. Now, it is the amount of water expelled or drawn in which is to be measured—this is accomplished by the following means: The rubber tube F opens into a glass tube G, which, bending at right angles, descends to the level a b of the liquid contained in the large beaker P. The descending limb is perfectly vertical, and securely fixed in that position by being fastened to an iron support. A thin-walled test-tube, M, about eighteen centimetres long, is suspended by two silk threads from the double wheel L, and balanced by a counter weight N, which also carries a kymographic pen, suitable for drawing a line upon a sheet of paper drawn past the point of the pen by a clock-work constructed for that purpose.
The test-tube M is so hung that its axis is traversed by the vertical descending limb of the glass tube G, and the test-tube can move up or down without striking the tube occupying its center.
Fig. 1.—Mosso's Plethysmograph.
If, now, the glass cylinder containing the arm and the tubes F and G are filled with water, and the arm begins to expand, then the water will be forced out into the test-tube M, which will therefore descend into the fluid in the beaker, P, raising, as it descends, the counter-weight and pen, N. If, on the contrary, the arm contracts, the water will be sucked back, the test-tube M will be lighter, the counter-weight will pull it back, and the pen will descend. Let us suppose that the point of the pen rests against a sheet of paper which is being drawn along horizontally with a constant velocity. As long as the pen is stationary it will draw a horizontal line; if it ascends, it will draw an ascending curve; when it falls, a descending curve. Such a line is technically termed a tracing. It shows the extent and duration of the motion, and exemplifies one of the many applications of the graphic method. The horizontal distances, since they are determined hy the rate at which the paper moves, correspond to the time. For example, if the paper moves an inch per minute, then, if we measure up a horizontal line the number of inches along which the tracing continues to rise, we can fix the points corresponding to a given time, and measure the height to which the pen had risen at any given instant, and also the rate at which it rose.
The vertical motion of the pen depends on the rise and fall of the test-tube M, which in its turn is caused by the out- and in-flow of the water surrounding the arm. Hence, a rise in the curve corresponds to an expansion, a fall to a contraction of the forearm. There are other details and precautions in the construction and use of the apparatus requisite to secure entire accuracy, for which I must refer to the original memoir. Modifications of the apparatus have been made by Ludwig and Kronecker, and a very essential improvement has been suggested and used by Dr. H. P. Bowditch, of Boston. These improvements are important to the experimenter, but have only a subsidiary interest for the general reader. The plethysmograph can be employed for many purposes. For example, it may serve as a very perfect thermometer, by applying it to record the changes of a closed volume of air, which of course increases or diminishes according as the temperature rises or falls.
With the aid of the plethysmograph Mosso discovered that the activity of the brain is directly connected with the circulation of the forearm. The importance of this fact will be better understood, if it is considered that we here have a physical phenomenon we can accurately measure, directly connected with mental phenomena we can not measure.
It is well known that the functional activity of an organ of the human body is accompanied by increased activity of the circulation, and this is also true of the brain. Since the total amount of blood is not subject to rapid and sudden changes, it is evident that, when an active part receives more blood, other parts must receive less. When the brain is at work, blood is withdrawn from the arm, which therefore becomes smaller.
In order that the volume of the forearm should remain constant, absolute a visit." I have translated as literally as possible Mosso's words—those unfamiliar with Italian should remember that words like "veneration" and "master," when rendered literally, convey a more fervid feeling in English than is expressed by the original. We reproduce the tracing referred to; the upper line is drawn by the pen, and its descent corresponds to the diminution of the volume of Mosso's right forearm; A marks the point when Professor Ludwig entered. The lower line records the time, each notch corresponding to an interval of five seconds. The diminution of volume was equal to about sixis necessary; the slightest movement of the mind suffices to disturb the equilibrium of the vascular system. The plethysmograph is an instrument with which we can record even emotions not depicted by any expression of the countenance, or revealed only by unnoticeable changes in the beating of the heart or of the respiratory movements. "Therefore," says Mosso, "I think it my duty to commence with a tracing that represents the sentiment of veneration which I felt in the presence of my beloved master. Behold, in fact, the contraction of the vessels produced by the entrance of Professor Ludwig, every time he honored the researches made upon myself with
Fig. 2.—Curve traced by the Plethysmograph, showing the diminution of size in the fore-arm, produced by the emotions caused by Ludwig's entrance at A.
cubic centimetres. This effect was not due to any strong emotion of fear or anxiety, but merely to the affectionate respect which Professor Ludwig's genial manners win from his pupils. The same experiment was tried upon Dr. Pagliani, with the same result.
Cerebral activity, like all the emotions, is reflected by the vascular system. To test this, the person experimented upon was given some simple arithmetical problem to solve; this was chosen because it could be solved without the person's speaking, which would have necessitated a change in the respiration, and so have modified the circulation. Fig. 3 shows the effect of calculating 245 X 15. The lines as before. This experiment was made upon Dr. Frey, and testifies to the great effect of a comparatively light and brief mental labor. It appeared that the
Fig. 3.—Curve traced by the Plethysmograph, showing the diminution of size in the fore-arm of Dr. Frey, produced by his calculating 245 x 15.
more complicated the sum to be solved the longer and greater was the contraction of the arm. The response of the circulation is very ready and marked, and, in persons of excitable temperaments, strong effects are produced from such very slight causes that it is sometimes difficult to make any experiments upon them.
There is undoubtedly a relation between the intensity of the emotion or mental process and the change in the circulation. This is illustrated in the following curious experiment upon one of Mosso's friends, who was a student of literature: While his arm was in the apparatus, Mosso presented to him a few pages on which were pasted paragraphs in Greek and Italian indiscriminately. By watching the changes in the volume of the arm, Mosso was able to decide correctly when his friend was reading a Greek paragraph, because to the greater mental effort corresponded a greater contraction of the vessels.
But the amount of blood in the extremities varies not only with psychical but also with physiological activities—for instance, with the respiration. The size of the forearm was shown by the plethysmograph to diminish during a deep inspiration, to increase during a prolonged and powerful expiration; or, again, alterations may be called forth by irritation of the skin of the arm, or even of a distant part of the body, or by direct compression of such of the veins or arteries of the upper arm as do not lie too deep to be reached, or even by changing the position of a limb other than the one being experimented upon. In brief, an almost endless variety of circumstances affect the circulation of a given part, as shown by the changes of its volume; but, among these circumstances, the condition of the brain is especially influential.
Mosso, however, has not confined his investigations to the waking condition, but has extended them also to men asleep, thus discovering a very great increase in the volume of the forearm as a person gradually falls asleep. The large size of the forearm during sleep may be diminished by a dream, or by any cause that renders the sleep less profound. It was evident that persons hear in their sleep various sounds, which disturb their slumbers but do not wake them up. When his friend was asleep, Mosso saw him move as a dog near by barked, and at the same time the apparatus recorded a diminution in the size of the extremity. Observation in this case shows that the cerebral activity during sleep is much greater than is usually supposed, and that a person may dream, as is evident by his moving or making some sound, yet have no recollection of it upon awaking. The plethysmograph preserves a more accurate record, for the slightest movement or disturbance produces its effect upon the arm, diminishing its volume. As a person awakes naturally, the size of the forearm is gradually lessened, because the blood is withdrawn.
To summarize the result: whenever the brain acts in any way, blood is withdrawn from the arm and from all the extremities; when the brain is inactive, more blood circulates in the limbs, most during sleep.By the same apparatus, Mosso also discovered that the circulation was changed by a dose of chloral very nearly, if not exactly, as in natural sleep; and that this drug, tested by these phenomena, produced a slumber very similar to normal sleep.
Pursuing his researches in other directions, Professor Mosso has succeeded in demonstrating that, when the brain acts, not only does the arm receive less blood, but the brain actually does receive more. This result was, of course, to be anticipated; and in making this further observation Mosso is not the first, for other physiologists have published researches upon this point, only less accurate and complete than those of the distinguished Italian investigator.
Opportunities for observing the circulation of the human brain are very rare, and occur only in consequence of violent accidents or insidious diseases, causing a loss of a piece of the bony skull sufficient to leave the soft brain exposed. Through such an opening it can be observed that the soft brain is smaller during sleep than during waking, because during the former state it draws back from the opening, and during the latter it may swell so much as even to protrude through the opening. That these changes are not abnormal results due to the diseased condition, is shown by the experiments which have been made upon healthy dogs by artificially removing a small piece of the skull, which can be done without causing any serious injury. In animals thus operated upon, the same changes of volume can be seen to occur in the brain, and closer observation shows that the variations are due to contraction or expansion of the blood-vessels.
These investigations demonstrate that one of the physiological conditions of increased mental action is an increased supply of blood, which is produced principally by a dilatation of the cerebral blood-vessels, accompanied by a contraction of the blood-vessels of other parts of the body. The measurable volume of the arm is thus partly a signal of the condition of the mind we can not measure, as affirmed in the early part of this article.
In connection with the new tendencies of psychology and physiology, such investigations as we have just described acquire a peculiar significance. The progress of knowledge has so enlarged the domains of both psychology and physiology, that they now overlap. The fields of investigation held in common form the bourn of "physiological psychology," as it is termed by the Germans, who are ever ready with a new name. Now, the mind derives its material through the senses. The sensations arise from physical causes. The final results of mental performances are various actions of the body, physical events such as motion and speech. Physics, therefore, are the alpha and omega of our mental history. Concerning what occurs between the physical cause of sensation and the physical result of mental action, two extreme opinions stand opposed. On the one hand, mind is defined as a succession of purely physical phenomena; on the other, as a supernatural and immortal power.
Hitherto psychologists have usually studied very little besides what we might call the natural history of the mind. Just as the ornithologist may study the habits of a bird, its mode of hopping, flying, feeding, singing, and so forth, and make himself thoroughly familiar with its natural history, without learning anything of its anatomy, the laws of muscular contraction or digestion, so also the philosopher may investigate the actions of the mind, the succession and relations of ideas, or may formulate the principles of logic, in entire ignorance of the processes which occur in the brain. The conclusions in both cases may be perfectly accurate, but they do not concern the more hidden and less accessible factors.
It is important to recognize the relation of psychology to the physiology of the brain, and to relegate both to their proper places. In reality they are only the different sides of one study and the best distinction of psychology is its success in arranging and classifying the psychic phenomena, whose relation to the physical basis of mind is to be determined. Although psychology is usually regarded as a department of philosophy, it is certainly more completely a natural science, since it deals with natural events, which are learned by direct observation, and which we coordinate by our reason. The slow but unmistakable gravitation of psychology toward physiology does not forecast, it seems to me, the demise of the former, but indicates rather that psychologists, having now gathered and arranged a great mass of data concerning the mind, are making an inevitable step in progress in seeking deeper than ever before for explanations. During the new phase, into which psychology has apparently entered, the principal problems will probably concern the relation of mind to the substratum of matter in which it displays itself. The most important steps which we can hope to take at present must, as far as we can judge, be taken in the field of physiological psychology, the essential purpose of which is to discover the exact nature of the dependence of the psychic phenomena on the physiological and anatomical properties of the body. It is precisely in this direction that Professor Mosso has made an important advance.
Mind appears to us as an image, dimly seen through the clouds of physical facts which encompass it. Some assert it to be merely a mirage, or, at most, an accidental shape into which physical facts have compiled themselves. Others believe that mind is a thing of its own kind. When the sunlight of discovery shall dispel the mists of the unknown, that conceal the true nature of the mind, then that image, now so dim, will become distinct, and its real character evident. That such a result is attainable is the belief, without which many laborious investigations would become purposeless.