dently terminate exactly when they have reached the lobule to which they are destined. The mucous membrane appears also to stop at this point; it is, at least, impossible to trace it on the vascular divisions which chiefly constitute the lobule. All the cells of one lobule communicate with each other, but not with those of adjacent lobules. On examining the cells particularly, Dr. Magendie says he was struck by the variation in size which they suffer in the progress of life: their number is in an inverse ratio with the years of the individual, and, consequently, each becomes more spacious as the subject increases in age. The specific gravity of the organ is so much lessened by this change, that a certain volume of the lung of a man seventy years old was found to weigh fourteen times less than the same volume of the lung of a child which had lived but a few days after its birth. The increase in the size of the cells is, in general, sufficiently regular to enable us (Dr. Magendie says,) to determine, very nearly, the age of the individual to which the organ has appertained. In old men, however, who have preserved a moderate degree of lustiness and a voluminous muscular system, in whom the heart has necessarily large dimensions, and where the quantity of blood is great, the pulmonary cells are not so much enlarged. The contrary is the case in emaciated, debilitated, old men, in whom the heart is almost always small, and the quantity of blood not very considerable.

This sort of rarefaction of the tissue of the lung by the progress of age, depends essentially on a diminution of the number of the capil. lary vessels by which the blood coming from the right ventricle of the heart passes into the pulmonary veins: it is a diminution of the surface where respiration is effected, that is to say, where the transformation of venous into arterious blood, and the pulmonary transpiration, take place. Such an obliteration of capillary vessels is only similar to what occurs in the body generally, the effects of which are, however, peculiar in this case, in consequence of the diminution in the extent of the function which they here perform.

It is the duty of an historian to notice refutations of errors which have been generally prevalent as truths, as well as to record new and original discoveries: some observations by Dr. Magendie on the lymphatic vessels of birds, will, therefore, engage a little of our attention on the present occasion.

Until the year 1768, as every anatomist will remember, the existence of lymphatic vessels of the digestive organs in birds was never affirmed: it was in that year that HEWSON announced his supposed discovery of them, in a letter to JOHN HUNTER, which was published in the 58th volume of the Philosophical Transactions. Hewson, on the same occasion, speaks of lymphatic vessels in the neck of birds; the discovery of which, he says, did not originate with him, but with John Hunter. Dr. Magendie says he was surprised-on reading this paper of Hewson's about three years since that so able an anatomist as John Hunter should have found such vessels in the neck, and have overlooked them in, the abdomen, had they really existed. This reflection, notwithstanding the apology of Hewson,-their being filled

with only pellucid and almost colourless chyle,-induced Dr. Magendie to make some researches on this subject. He chose the goose for the subject of his examinations, because it was in this bird that Hewson states he had discovered them. He readily found them, on each side, in the neck; but "he was not a little surprised (he says) at not per. ceiving any trace of lymphatic vessels in the mesentery, notwithstand. ing the care and attention with which the examination was made.” He sought for the considerable plexus which, according to Hewson, embraces the superior mesenteric artery; but he was not more suc cessful; and it was in vain that he endeavoured to find the double thoracic duct. He repeated his examination a second and a third time, increasing his care and precautions; but it was useless: he always readily found the veins in the neck, but never any trace of "chyliferous vessels, nor of the thoracic duct." He repeated his researches during digestion, but they were equally fruitless. He has since dissected above fifty birds, " of all kinds," carnivorous and others, and examined them whilst digestion was going on. The result of the whole of his observations has led him to infer, that "the chyliferous vessels and the thoracic ducts do not exist in birds, and that the only traces of lymphatic vessels are seen in the neck."—" What anatomical cir. cumstances, then, (Dr. Magendie says,) can have deceived Hewson, and led him into so serious an error as that into which he has fallen ?" If it related only to the chyliferous vessels, he adds, the circumstance might be explained by supposing that Hewson had mistaken nerves for them; but this will not account for what he says about the thoracic ducts, which he states that he had even injected. There is no vessel running from the abdomen to the subclavian veins: even the azygos is absent in birds. The only structures which he thinks can have been mistaken for the thoracic ducts by Hewson, are the arterial tubes which sometimes "run from the middle part of the aorta to the pulmonary arteries, which are connected with the subclavian veins; but, as these vessels are entirely obliterated a few days after birth, it still remains to be known how Hewson could have injected them."

The foregoing remarks were comprised in a Memoir read to the Academy of Sciences in 1819: since that time Dr. Magendie has dissected a great number of birds, and has not only confirmed his former observations, but has also ascertained that the swan and the goose are the only birds which have a lymphatic apparatus on each side of the neck, terminated by a sort of lymphatic gland which communicates in some cases immediately with the subclavian vein.

Dr. Magendie has commenced some researches relative to the lymphatic vessels of reptiles and fish, described by Hewson and Monro. Hitherto his observations have led him to infer that these animals are entirely devoid of lymphatic vessels, and that the organs described under this name are only "sanguiferous veins." The sea-turtle, he adds, is the only one of those animals which appears to furnish objections to the foregoing as a general inference. He has himself in. jected vessels in the mesentery of this animal, which had the disposition of lymphatic vessels; but, as the mesentery was detached, it was impossible for him to follow them towards the vertebral column,

and, consequently, to assure himself that they were really lymphatic vessels."

The latter observations, if strictly correct, are especially interesting in regard to a question which is still the subject of dispute amongst eminent physiologists, that of the mechanism by which absorption is effected. A French physician, of considerable talents and much eru. dition,* has endeavoured to settle this point by some conclusions chiefly founded on his interpretations of certain phenomena of the human body, rather than on positive evidence. His disquisition is noticed in this place, although it more properly appertains to physi ology, because his explanation rests on the supposition that there is in the human body a series of vessels which have not hitherto been physically demonstrated, and which are said by him to originate in the intimate structure of the textures and terminate in veins.

The researches of several anatomists of the last and the preceding century, especially those of HALLER, SCARPA, KING, RUYSCH, LEUWENHOECK, BLANCARDI, and VIEUSSENS, have made it apparent, as well as we can be informed by sensible evidence, that the ultimate arrangement of the structure of the human body is vascular; and it has been proved that many parts, apparently of a simple laminous or fibrous texture, possess vessels of a considerable size, which are ordinarily devoid of fluids, or contain only such as are pellucid and colourless, as well as the vessels themselves, and which are therefore imperceptible in the natural state, though they may be filled with fluids by artificial injections from the arteries, and by red blood under cer. tain vital conditions. The existence of a series of vessels, not recognizable under natural or ordinary circumstances, having a connexion with the arteries, and canals continuous with those of the latter vessels, -is clearly evident; but the manner of this connexion is a point which has not been so satisfactorily determined. The greater part of modern anatomists, with BICHAT, whilst they have admitted that some of the extreme divisions of arteries are immediately continuous with ramifications of veins, have believed that other divisions of arteries terminate in the pellucid vessels above designated, and which have been ordinarily termed capillary or exhalant vessels. Dr. ALARD (who is alluded to in the foregoing paragraph,) endeavours to prove that the ramifications of the arteries are every where continuous with those of the veins, and that the pellucid capillary or exhalant vessels are given off from the parietes of the arterial canals in various parts of their course.

This is a point which can be settled only by observations and experiments instituted with that express view, and it is not of essential importance in regard to our present object, which is to state that Dr. Alard believes that there are not less forcible arguments for the existence of a similar series of vessels connected in the same manner with

Dr. ALARD. See his treatise Du Siége et de la Nature des Maladies; ou, nouvelles Considerations touchant la veritable Action du Système Absorbant dans les Phenomènes de l'Economie Animale. 2 tom. 8vo. Paris, 1821.

the veins: the former carrying the fluids intended for nutrition, the secretions, and exhalation; the latter, performing the functions of absorption. Some of these vessels, whose open mouths are present every where, in the most intimate textures of the organs as well as on the surfaces of the great cavities, are supposed by Dr. Alard to terminate in the parietes of the adjacent veins; whilst others unite and form the trunks which are generally known by the name of absorbents. The chief arguments adduced by Dr. Alard in support of his proposition of the existence of a series of pellucid vessels terminating in adjacent veins, are the results of some injections by several anatomists, who appear to have filled what were called lymphatics, by injecting veins as well as arteries and absorbent trunks; so that the same vessels which were termed arterious by RUYSCH, and lymphatic by CRUIKSHANKS and MASCAGNI, are considered as venous by Dr. RIBES. This eminent pathologist has shownt that the superficial lymphatics of the liver may be filled by injecting the hepatic veins. The result was the same in several repetitions of the experiment. Dr. Ribes had previously been led to conjecture that the lymphatics in the medullary cavities of the bones, that absorb the marrow, terminate in veins. The experiments alluded to satisfied him that all the vessels having the character of lymphatics do not run into the thoracic duct. The recent discovery of Dr. FоHMANN, of Heidelberg, (noticed in the last Proëmium,) -of a communication of the lymphatics of the intestines with the mesenteric veins, in some animals,-concurs, with the above-mentioned facts, to support the proposition of Dr. Alard; but the arguments hitherto adduced in its favour are certainly not sufficient to establish it in a satisfactory manner, in its application to the system generally. The view of Dr. Alard-that supposing the existence of minute pellucid vessels, springing from the parietes of the small arteries; distributed in every part of the body; conveying different fluids, and producing different effects, according as their vital properties are mo. dified; having corresponding vessels, which spring from the most intimate texture of the organs and the surfaces of the great cavities and unite in larger tubes, forming in some instances long continuous canals, (ordinarily denominated absorbents,) in others running to be inserted into veins,-is one which is qualified to explain, more plausibly than any other, the mechanism of the distribution of the fluids for the purposes of the organic functions; and is, besides, capable of obviating the difficulties which have been presented by the diversity of the results of the experiments of HUNTER, Dr. MAGENDIE, Mr. BRODIE,

Although these vessels perform such an office, Dr. Alard thinks they should be regarded as absorbents. He considers that the way by which they obtain the fluids they convey is similar to that by which the absorbents (properly,) obtain theirs; and that both series of vessels, as well as that which he supposes to be connected with the minute veins, are endowed with similar properties and go. verned by similar laws, which laws and properties have nothing strictly analogous to those of the blood-vessels; and, hence, that BICHAT, in making his capillary system a division of the arterial, was in error.

+ Memoires de la Société Medicale d'Emulation, tom. viii.

and several other eminent physiologists, relative to the mechanism of absorption: but the history of the sciences presents so many instances of postulates, having the greatest plausibility at one time, being proved invalid at a subsequent period; and of hypotheses founded on such data that have explained the objects to which they relate with as much apparent perspicuity, that we cannot be too strict in refusing implicit assent to a theory constituted like that of Dr. Alard. This doctrine, however, with the discovery of Dr. Fohmann, and the results of the injections of Dr. Ribes, will perhaps induce some anatomists to institute researches, with the object of ascertaining whether or not there is an immediate communication of veins with lymphatics in the body generally, as well as in the liver and the mesentery.

A species of mucous follicle, situate immediately beneath the skin, in several parts of the body, and not hitherto described, has been very recently observed by Professor ScHREGER, of Erlangen. These follicles, or, as he terms them, subcutaneous mucous bursæ, are formed by the lamellæ of the adipose cellular membrane, and are present in the region of all the joints subject to powerful fluxions and extensions; but especially about those of the knee, the arm, and the first and second series of phalanges of the hands and feet. It is only on the anterior surface of joints, or that where the skin is rendered tense from the motion of the limb, that they are situate. Their figure varies in different parts, being round, oblong, double, triple, &c. They contain a fluid which is, apparently, poured out on the surface of the skin, for its lubrication.

There is nothing else to be noticed relative to the anatomy of the human body, nor are there any recent discoveries in Comparative Anatomy, that tend, in a remarkable degree, towards its illustration. The only important observations in this department of natural history, are those of Mr. AUDOUIN, on the structure of the thorax of insects, and those comprised in the account of the dugong, by Sir STAMFORD RAFFLES; which, as already indicated, are too remotely related with the elements of medicine to be, with propriety, detailed in the present essay.

* The reader is referred to the London Medical and Physical Journal of May, 1821, for a more detailed account of Dr. Alard's observations and opinions.

+ It may be proper to state, BLUMENBACH has published Decas sexta Collectionis suæ Craniorum diversarium gentium, (Gottingæ ;) but it cannot be considered a fit subject for a particular account in this Essay.

In a memoir read to the Academy of Sciences of Paris, Feb. 19, 1821. § Published in the Philosophical Transactions for 1820.

The observations of Mr. JAMES WHITE, of Bath, on the Udder of the Cow, which were published in the London Medical and Physical Journal for June 1821; and on the Thyroid Gland in the Horse, (same work, Number for May;) are not sufficiently precise for the purposes of this Sketch. It may, however, be allowable for the writer to state, that Mr. White believes he has ascertained that the thyroid of the horse is a mucous gland, and that its excretory ducts open into the cavity of the trachea. He has adduced several observations tending to show the probability of a similar structure and function in this organ in man. His ob