observed, it appears to have lost all its muscular energy. Its entire form would indicate great swiftness of motion, which is doubtless effected by means of the same interesting sinuous inflections in the water, which excite our admiration in the class of serpents upon land; as it is indeed well ascertained, that some serpentiform fishes, especially the true murænæ, are capable of transferring with great effect, their aquatic locomotive powers to the surface of the earth. In what manner its enormous pouch is employed in its economy, I repeat, that I cannot presume to determine; its complete. inflation with air, when on the surface, must, it would appear, afford a very effectual obstacle to the descent of the animal beneath. Whether it be capable of secreting the contained air, as has been thought of the common internal air vesicles of some other fishes, for which apparent purpose, eels have a peculiar gland connected with its centre, or whether water be allowed to enter its cavity, as it has been thought to enter the abdominal cavity of some rays, would form a subject of interesting enquiry. Having however entered on the field of conjecture, I shall not presume to longer engage the attention of the Royal Society, though I cannot but express a hope, that the discovery of another curious link in the vast chain of being, and especially in latitudes where such an one might have been least expected, will be considered sufficiently important to merit their notice.

VI. An examination into the structure of the cells of the human lungs; with a view to ascertain the office they perform in respiration. By Sir EVERARD HOME, Bart. V. P. R. S. Illustrated by microscopical drawings from the pencil of F. BAUER, Esq. F. R. S.

Read February 8, 1827.

No subject connected with physiological enquiry has more

excited the attention of the anatomist and chemist, than respiration; but the association between this subject and animal heat, which has so long been supposed to exist, has led to the belief, for the last century, that both enquiries belong more particularly to chemistry than anatomy, and I may probably be considered as going out of my province in taking up this investigation. On the other hand, I see reason to believe that the process of respiration is in itself more simple than is imagined, and more within the reach of discovery by means of accurate anatomical knowledge of the parts. employed, than by means of acquaintance with the intricacies belonging to chemical affinities: I carry this so far as to contend that no explanation of respiration upon chemical principles is to be depended on, unless it accord in all respects with the anatomy and physiology of the lungs, by which the assumed process takes place.

The present theory respecting respiration adopted by the chemists, is, that this process decarbonises the blood in the following manner; at every inspiration a compound of

oxygen and nitrogen, mixed together, is received into the lungs, and in every expiration, the same volume is returned, measure for measure exactly, with this only difference, that what entered as oxygen is returned in the form of carbonic acid gas, which, according to their theory, proves that no part of the inspired atmospheric air has been retained in the lungs, but a quantity of carbon, equal to that of the oxygen inspired, has been extracted from the blood by the oxygen, making it become carbonic acid gas.

Nothing could be more ingenious than this theory, were it supported by the structure of the lungs themselves, and it could be proved that the blood requires no other changes for its purification; since all the leading facts on which it is founded, are completely established upon the firm basis of experiment.

When this theory was formed, the structure of the air cells of the lungs had never been examined, the more minute structures in animal bodies being at that time considered beyond the reach of examination; and it is the object of the present communication, to bring forward an explanation of the mechanism of the cells of the lungs, as well as of the different distributions of the vessels that ramify through those organs, acquired from Mr. BAUER'S microscopical observations, and to see how far they are fitted for the process, which by this theory is allotted to them.

In this investigation, I began by an enquiry into the circulation of the blood through the lungs, in the labour of which I have been very ably assisted by Mr. RUSSELL, a very intelligent student of St. George's Hospital, at present filling the highly respectable and important office of senior House

Surgeon. To him I am indebted for having taken the trouble of making injections of the arteries, the veins, and of the cells of the lungs, with different substances, so as to enable Mr. BAUER to expose and examine them on the field of the \microscope.

The first new fact discovered in the course of this enquiry was, that although the common minute injection used by anatomists for filling the blood vessels, when thrown in by the trunk of the pulmonary artery, while the cells of the lungs are empty, returns again by the trunks of the pulmonary veins, yet when thrown in by the veins, it is not returned by the trunks of the arteries.

Another fact was discovered; that during the momentary distention of the air cells, an interruption is produced between the arterial and venal circulation in the lungs, the blood being carried no farther than the small arterial branches surrounding the air cells.

The following description of the air cells, and the parts surrounding them, is taken from the annexed microscopical drawings of Mr. Bauer.

As accurate representation surpasses all verbal description, I shall not have occasion to do more, to make myself understood, than to mention the parts themselves, and the circumstances under which they are represented.

The cells of the lungs were filled with quicksilver, to show their utmost capacity, and the parts were afterwards immersed in rectified spirit, to prevent the cells from collapsing, when the quicksilver was allowed to escape.

When the internal cavity of a single cell was exposed, immediately behind its internal membrane, the branches of