tlemen remark, that it can only be adopted with perfect safety in animals of like kinds, or in those the corpuscles of whose blood are of similar configuration. MM. Prévost and Dumas, Dieffenbach,' and Bischoff, all agree as to the deadly influence of the blood of the mammalia when injected into the veins of birds. This influence, according to Müller, is in some way connected with the fibrin of the blood, and experiments have certainly shown, that blood deprived of fibrin acts most injuriously when injected into the vessels.3

The introduction of the practice of infusing medicinal agents into the blood was coeval with that of transfusion. It appears to have been first subjected to a philosophical examination by Sir Christopher Wren, who practised it on a malefactor in 1656.4

5

It is a singular fact, that in cases of infusion, medicinal substances are found to exert their specific actions upon certain parts of the body, precisely in the same manner as if they had been received into the stomach. Tartar emetic, for example, vomits, and castor oil purges, not only as certainly, but with much greater speed; for, whilst the former, as before remarked, requires to be in the stomach for fifteen or twenty minutes, before vomiting, is excited, it produces its effect in one or two minutes, when thrown into the veins. Dr. E. Hale, of Boston, has published an interesting pamphlet on this subject. In it he traces the history of the operation, detailing several interesting experiments upon animals; and one upon himself, which consisted in the introduction of a quantity of castor oil into the veins. In this experiment, he did not feel much inconvenience immediately after the injection; but very speedily experienced an oily taste, which continued for a length of time, and the medicine occasioned much gastric and intestinal disturbance, but did not act as a cathartic. Considerable difficulty was experienced in the introduction of the oil, to which circumstance M. Magendie ascribes Dr. Hale's safety; for it is found, by experiments on animals, that viscid fluids, such as oil, are unable to pass through the pulmonary capillaries, in consequence of which the circulation is arrested, and death follows. Such, also, appears to have been the result of the experiments of Dr. Hale with powdered substances.

The injection of medicines into the veins has been largely practised at the Veterinary School of Copenhagen, and with complete successthe action of the medicine being incomparably more speedy, and the dose required much less. It is rarely employed by the physician, except in experiments on animals; but it is obvious, that it might be had recourse to with happy effects, where narcotic and other poisons have been taken, and where the mechanical means for their removal are not at hand.

'Die Transfusion des Blutes, Berlin, 1828.

2 Müller's Archiv., 1835; cited in Baly's translation of J. Müller's Handbuch, u. s. w. 3 See, on the different effects of transfusion of arterial and venous blood on animals, Bischoff, in Müller's Archiv., Heft iv. 1838, cited in Brit. and For. Med. Rev., April, 1839, p. 548. ♦ Chelius, System of Surgery, translated by South, Amer. edit., iii. 626, Philad., 1847. 5 Boylston Medical Prize Dissertations, for the years 1819 and 1821, p. 100, Boston, 1821. • Précis, &c., ii. 430.

4. CIRCULATORY APPARATUS IN ANIMALS.

In concluding this subject, a brief allusion to the circulatory apparatus of other parts of the animal kingdom may be interesting and instructive.

In the mammalia in general, the inner structure of the heart is the same as in man; but its situation differs materially; and in some of them, as in the stag and pig, two small flat bones, called bones of the heart, exist, where the aorta arises from the left ventricle. In the amphibious mammalia and the cetacea, it has been supposed, that the foramen ovale in the septum between the auricles is open as in the human foetus, to allow them to pass a considerable time under water without breathing; but the observations of Blumenbach, Cuvier, and others seem to show, that it is almost always closed. Sir Everard Home found it open in the sea otter, in two instances; but these are regarded by naturalists as exceptions to the general rule. In several of the web-footed mammalia and cetacea, as in the common otter, sea otter, and dolphin, particular vessels are always greatly enlarged and tortuous;-a structure which has been chiefly noticed in the vena cava inferior, and is supposed to serve the purpose of a diverticulum, whilst the animal is under water; or to receive a part of the returning blood, and retain it until respiration can be resumed.

In birds, the structure of the heart universally possesses a singular peculiarity. Instead of the right ventricle having a membranous valve, as in the left, and as in all the mammalia, it is provided with a strong, tense, and nearly triangular muscle, which aids in the propulsion of the blood from the right side of the heart into the lungs. This is presumed to be necessary, in consequence of their lungs not admitting of expansion like those of the mammalia, and of their being connected with numerous air-cells.

The heart of reptiles or amphibia in general consists either of only one ventricle, or of two, which freely communicate, so as to constitute essentially but one. The number of auricles always corresponds with that of the ventricles. That the cavities--auricular and ventricularare, however, single, although apparently double, is confirmed by the fact, that, in all, there is only a single artery proceeding from the heart, which serves both for the pulmonic and systemic circulations. After this vessel has left the heart, it divides into two branches, by one of which a part only of the blood is conveyed to the lungs, whilst the other proceeds to different parts of the body. These two portions are united in the heart, and after being mixed together are sent again through the great artery. In these animals, therefore, aeration is less extensive than in the higher; and we can thus understand many of their peculiarities;-how, for example, the circulation may continue, when the animal is so situate as to be incapable, for a time, of respiration; as well as the great resistance to ordinary deranging influences, by which they are characterized. Fig. 304 represents the circulatory apparatus of the frog; in which E is the ventricle and D the auricle. From the former arises the aorta F, which soon divides into two trunks. These, after sending branches to the head and neck, turn.

downwards, (0 and P,) and unite in the single trunk A. This vessel sends arteries to the body and limbs, which ultimately terminate in veins, and unite to form the vena cava C. From each of the trunks into which the aorta bifurcates at its origin arise the arteries F. These are distributed to the lungs, and communicate with the pulmonary veins, which return the blood to the auricle, D, where it becomes mixed

with the blood of the

Fig. 306.

systemic circulation. In the tadpole state, the circulation is branchial, as in fishes. The heart then sends the whole of its blood to the branchia or gills, and it is returned by veins following the course of the dotted lines M and N, (Fig. 304,) which unite to form the descending aorta. As the lungs undergo their developement, small arterial branches arise from the aorta and are distributed to those organs; and in proportion as these arteries enlarge, the original branchial arteries diminish, until ultimately they are obliterated, and the blood flows wholly through the enlarged lateral trunks, O and P, which, by their union, form the descending aorta.

In fishes, the heart is extremely small, in proportion to the body; and its structure is simple; consisting of a single auricle and ventricle D and E (Fig. 305). From the ventricle E an arterial d trunk arises, which, in most fishes, is expanded into a kind of bulb, F, as it leaves the heart, and proceeds straight forward to the branchia or gills, G and H. From these, the blood passes into a large artery, A, analogous to the aorta, which proceeds along the spine, and conveys the blood to the various parts of the system; and, by the vena cava, C, the blood is returned to the auricle. This is, consequently, a case of single circulation. Insects appear to be devoid of blood vessels. Cuvier examined all the organs in them, which, in red-blooded animals, are most vascular, without discovering the least appearance of a bloodvessel, although extremely minute ramifications of the trachea were obvious in VOL. II.-13

Interior of the Leech.

a, a. Respiratory cells.

b. Two large arteries. Glands connected with the f Penis. g. Uterus. (Sir E. Home.)

e, c. Mucous glands. d, d.

testicles. e, e. Testicles.

every part. Insects, however, both in their perfect and larve state, have a membranous tube running along the back, in which alternate dilatations and contractions are perceptible, and which has been considered as their heart; but it is closed at both ends, and no vessels can be perceived originating from it. To this the innumerable ramifications of the trachea convey the air, and thus, as Cuvier has remarked, "le sang ne pouvant aller chercher l'air, c'est l'air qui va chercher le sang;" ("the blood not being able to go in search of the air-the air seeks the blood.") Carus, however, discovered a continuous circulation through arteries and veins in a few of the perfect insects, and especially in some larves. Lastly: in many genera of the class vermes, particularly amongst molluscous animals, there is a manifest heart, which is sometimes of a singular structure. Some of the bivalves-it is affirmed-have as many as four auricles; whilst many animals, as the leech and Lumbricus marinus, have no heart; but circulating vessels exist, in which contraction and dilatation are perceptible.

The marginal figure, (Fig. 306,) of the interior of a leech, given by Sir Everard Home, exhibits the mode of circulation and respiration in that animal. There is no heart, but a large vessel exists on each side. The water is received, through openings in the belly, into the cells or respiratory organs, and passes out through the same.

CHAPTER V.

NUTRITION.

THE investigation of the phenomena of the circulation has exhibited the mode in which arterial blood is distributed over the body in minute vessels, not appreciable by the naked eye, and often not even with the microscope, and so numerous, that it is impossible for the finest-pointed instrument to be forced through the skin without penetrating one, and perhaps several. It has been seen, likewise, that, in the capillary system of vessels, this arterial blood is changed into venous; and it was observed, that in the same system, parts are deposited or separated from the blood, and certain phenomena occur, into the nature of which we have now to inquire; beginning with those of nutrition, which comprise the incessant changes that are taking place in the body, both of absorption and deposition for the decomposition and renovation of each organ. Nutrition is well defined by M. Adelon' as the action, by which every part of the body, on the one hand, appropriates or assimilates to itself a portion of the blood distributed to it; and, on the other, yields to the absorbing vessels a portion of the materials that previously composed it. The precise character of the apparatus, by which this important function is accomplished, we have no exact means of knowing. All admit that the old matter must be taken up by absorbents, and the new be deposited by arteries, or by vessels continuous with them. As the precise arrangement of these minute vessels is not perceptible by

Physiologie de l'Homme, tom. iii. p. 359, 2de édit., Paris, 1829.

the eye, even when aided by powerful instruments, their arrangement has given rise to controversy. Whilst some have imagined lateral pores in the capillaries, for the transudation of nutritive deposits; others have presumed, that inconceivably small vessels are given off from the capillary system, which constitute a distinct order, and whose function is to exhale the nutritive substance,-an idea, which, as has been said elsewhere, has been revived by M. Bourgery. Hence, they have been termed exhalants or nutritive exhalants; but the anatomical and physiological student must bear in mind, that whenever the term is used by writers, they do not always pledge themselves to the existence of any distinct set of vessels, but merely mean the minute vessel, whatever may be its nature, which is the agent of nutrition, and conveys the pabulum to the different tissues.

In investigating the physiology of nutrition, two antagonistic processes demand attention; 1st. Decomposition, by which the tissue yields to the absorbing vessels a portion of its constituents; and 2dly. Composition, by which it assimilates a part of the arterial blood that enters it, and supplies the loss it had sustained by the previous act of decomposition. The former of these actions obviously belongs to the function of absorption; but its consideration was deferred, in consequence of its close application to the function we are about to investigate. It comprises what is meant by interstitial, organic, or decomposing absorption, and does not require many comments, after the long investigation of the general phenomena of absorption into which we entered. The conclusion, then arrived at, was,-that the chyliferous and lymphatic vessels form only chyle and lymph respectively, refusing the admission of all other substances;-that the veins admit every liquid which possesses the necessary tenuity; and that whilst all the absorptions,-which require the substance acted upon to be decomposed and transformed,are effected by the chyliferous and lymphatic vessels, those that demand no alteration are accomplished through the coats of the veins by imbibition. It is easy, then, to deduce the agents to which we refer the absorption of decomposition. As it is exerted on solids, and as these cannot pass through the coats of the vessel in their solid condition, it follows that other agents than the veins must accomplish the process; and, again, as we never find in the lymphatic vessels any thing but lymph, and have every reason to believe, that an action of selection is exerted at their extremities, similar to that of the chyliferous vessels on the heterogeneous substances exposed to them, we naturally look to the lymphatics as the main, if not the sole, organs concerned in the absorption of solids.

It has been maintained, by some physiologists, that the different tissues are endowed with a vital attractive and elective force, which they exert upon the blood;-that each tissue attracts only those materials of which it is itself composed; and thus, that the whole function of nutrition is an affair of elective affinity; but this, obviously, cannot be the force that presides over the original formation of the tissues in the embryo. An attraction cannot be exerted by parts not yet in existence.

1 See page 92 of this volume.