Influences which modify Functional Secretion. 109 similar and coherent particles, which is termed a conglomerate gland. From each separate molecule of the gland, one excretory tube at least issues, the connexion of which with the arteries and veins of the part has hitherto eluded satisfactory observation. In general the excretory tubes of a conglomerate gland coalesce to form a common trunk. Each elementary portion of such a gland resolves itself into blood vessels and excretory tubes. Upon the whole, it appears that a vascular membrane is all which is requisite for secretion, and that the other contrivances, which have been described, are but methods of conveniently packing a large extent of surface in a small compass. Functional secretion is remarkably under the influence of the nerves. Upon one affection of the mind the tears flow, upon a second the urine, upon another the saliva: yet I found upon cutting the nerves of the kidney in a dog, that in half an hour afterwards a quantity of urine had accumulated in the pelvis of the kidney, and in the ureter, which had been tied. In this as in the former mode of secretion, physical impressions locally applied, seem to have an influence upon the rate of its production. By the removal of the young from its mother the secretion of milk is after a short time entirely suspended, in circumstances where the gland would otherwise have continued its action for an indefinite length of time1. The influence of medicines tells immediately upon functional secretion. Many classes of drugs derive their names from the power which they possess of increasing i Bostock. Elements of Physiology, vol. ii. p. 404; and Quarterly Journal, vol. i. p. 165, 6. 110 Where does Secretion originate? the flow of saliva, of the urine, and the like; and their value, from the connexion which exists between the rate of a secretion and the conditions of other functions. Mr. Brodie ascertained that the secretion of urine does not take place in animals, in which after decapitation the circulation of the blood is sustained by an artificial respiration k. Three suppositions present themselves, as to the place in which the secretions are formed: either they may be produced in the blood while circulating in the system at large, and be simply separated through the intervention of secreting organs,—or the entire production of a secretion from the blood may be effected by the capillaries in each part, or the elements of different secretions may spontaneously develope themselves to a certain extent in the blood at large, yet require the influence of the capillary tubes in the part where they are separated, for their complete elaboration. The only fact with which we are acquainted upon this subject is, that after both kidneys have been removed, an animal survives several days, during which the characteristic element of the urine accumulates in considerable quantity in the blood, according to the observations of MM. Prévost and Dumas. III. When the resemblance was ascertained between the effects of combustion and of respiration upon atmospheric air, the lungs, which were previously supposed to act in cooling the heart, were invested by physiologists with the office of producing animal heat. The difficulty of accounting on this supposition for the equal diffusion of warmth throughout the body was evaded by, and served * Phil. Trans. vol. ci. p. 48. Heat liberated when the Blood becomes venous. 111 to confirm the beautiful theory of Crawford. By careful experiments Crawford became satisfied that arterial blood has a greater capacity for heat than venous blood in the ratio of 114.5 to 100. The heat therefore liberated in the lungs during respiration might become instantly latent, and form an unobserved element of arterial blood in its flow through the body; while at the subsequent conversion of arterial into venous blood in the capillaries, the heat would become liberated equally throughout the system. Numerous observations, which have successfully established that the vital heat in different animals, in the same individual, and even in plants, has a close relation to the quantity of oxygen consumed, seemed to place the theory of Crawford beyond the reach of innovation. Recent inquiry concurs with former experience upon the point before us. Dr. Edwards has ascertained that young animals consume in proportion less oxygen than adults, and have a less power of generating heat; and that young animals differ among each other in the power of producing heat, something in the ratio of the oxygen which they destroy. Where respiration is imperfect, as in asthmatic patients, the temperature of the body is lower. Where pure arterial blood does not circulate through the body, as in those, in whom a communication exists between the right and left cavities of the heart, the temperature is below the usual standard. But the experiments of Mr. Brodie have shown the preceding evidence to be fallacious, and prove that Crawford must have overrated the difference in the capacity for heat of arterial and venous blood, upon which his theory rested. Two rabbits, as nearly alike as possible, were destroyed by decapitation after securing the 112 Quantity thus produced comparatively trivial. vessels in the neck: in one the circulation was kept up by means of an artificial respiration; the other was left untouched in the same room at the same temperature. Of these two dead rabbits the first was observed to cool more rapidly than the second: yet in the first the chemical influence of respiration was perfectly sustained, the blood circulating through the lungs from a dark hue assumed the arterial character, that circulating through the body became venous; and the air respired was deteriorated exactly as by the breathing of a living rabbit. Nevertheless, heat was not derived in sufficient quantity from this source to make up for the lowering of temperature produced by the fresh draughts of cool air into the lungs of the dead animal: the thermometer at the expiration of thirty minutes stood at 97° in the rectum of the first, at 98° in the rectum of the second rabbit'. Subsequent researches upon this subject by Dr. W. Philip and Dr. Hastings tend to show, that the rapid cooling of the first animal in the experiment detailed may have resulted in part from too large an inflation of the chest ; that upon avoiding this excess, the process of cooling appears even to be retarded by artificial respiration, but not to a degree that invalidates the conclusiveness of Mr. Brodie's experiments. But although the theory of Crawford be thus set aside, it remains possible that arterial blood may prove by some other method the source of animal heat. A general ratio seems to exist between the temperature of parts and the afflux of arterial blood; and the following experiment by Dr. Wilson Philip may serve to show how the de 'Phil. Trans. vol. ci. p. 36, et seq.; and vol. cii. p. 380. Opinions respecting the Source of Animal Heat. 113 composition of the latter through an agency, in many instances analogous to the nervous influence, may produce heat. Upon applying the galvanic influence to arterial blood immediately upon being drawn, an evolution of heat amounting to 2° or 3° took place, while the blood assumed a venous hue. The trial was made with the arterial blood of a rabbit; the rise of température ceased to show itself in two minutes after the blood began to flow from the vessel, but the change in colour continued to be produced accompanied with an extrication of gas through the galvanic influence. No rise of temperature could be produced in venous blood by the same meansTM. The present opinion of physiologists inclines to the belief that the production of animal heat depends upon the nervous influence: yet the best evidence which we possess shows only that temperature may be modified through the nerves, like every other physical endowment of the body. Sir Everard Home found that upon the division of the nerves distributed to the growing antler, its temperature fell immediately several degrees, but rose again a few days afterwards even higher than the temperature of the opposite horn". Sir Everard mentions on the same occasion some curious instances of a partial extrication of heat, which he refers to nervous agency: the oviduct of a frog ready to spawn is two degrees hotter than the heart; and it appears on the authority of Dr. Granville, that during labour the heat of the uterus is sometimes raised to 120°; but a very similar phenomenon has been observed to occur in plants in which no m " Inquiry, &c. p. 242. I "Phil. Trans. vol. cxv. p. 7. |