The lymphatic or conglobate glandst com pose a very important part of the absorbent system, if we may judge from their number and their general diffusion over every part of the body. They are of various sizes, and are grouped together in various ways; occasionally they are single, but more frequently connected masses of considerable extent. They are found in almost every part of the body, always connected with the lacteals and lympha tics, and it is asserted that each one of these ves sels, in some part of its course, passes through or is connected with one or more of these glands.r. There are certain parts of the body in which they are more numerous, and are connected in larger masses ; the lacteals are furnished with numerous glands in their passage along the mesentery, while the glands that belong to the lymphatics are found in the greatest number and largest masses in the groin, the axilla, and the neck. It is necessary to remark that this account of the distribution of the lymphatic glands applies only to the animals which belong to the class of the mammalia; in birds they are much more rare, and still more so in fish, while among the lower animals, where we have suffi cient evidence of the existence of an absorbent system, the glands have not yet been satisfac torily demonstrated.§ With respect to the structure of these glands, as well as that of glands of other descriptions, a controversy has long existed among anato mists, whether they consist of a series of cells or follicles, as they have been termed, or whe ther they are composed simply of a congeries of vessels. The question may be regarded as still at issue ; but it may be remarked that whereas the older anatomists generally leaned to the opinion of the. follicular structure of the glands the moderns have more frequently adopted the hypothesis of their vascular texture, so that we may consider this doctrine as sup ported by the most recent and elaborate re searches.t See LYMPIIATIC ; GLAND.
§ 2. The question of venous absorption con sidercd.—We have now been describing those organs, which are more specifically or appro priately termed the absorbent system, as being those parts the office of which is confined to this operation. But a very important and in teresting question must now be discussed, whether the function of absorption is exclusive ly performed by the lacteals and the lymphatics.
The ancient anatomists and physiologists being unacquainted with the existence of the lacteals and the lymphatics, yet observing the evident effect of the operation of absorption, ascribed these effects to the action of the veins; and among the moderns, for some time after the discovery of what were more appropriately termed the absorbent vessels, it was still sup posed that the veins co-operated with them, and in some cases were even the principal agents. This was the universal doctrine until the middle of the last century, and was one of the points which was decidedly maintained by Haller and his disciples.T The arguments by which the hypothesis of venous absorption was supported may be re duced to two classes, partly of a physiological and pathological, and partly of an anatomical nature ; the first consisting of the results of experiments performed for the express purpose of investigating the subject, and of considera tions derived from the morbid conditions of the system; the second depending more exclu sively upon anatomical considerations. The
experiments referred to consisted in passing injections from the veins to the absorbents, or the reverse, thus proving, as was supposed, that a direct connexion subsisted between these vessels. They were performed by the most skilful anatomists of the age, and were gene rally acquiesced in, without either the accuracy with which they were conducted, or that of the conclusions deduced from them, being ever called in question. Another class of experi ments consisted in passing ligatures round the thoracic duct, so as to render it impervious to the passage of the chyle, when it was supposed that under these circumstances the nutrition of the animal was not interrupted,''' and the same conclusion appeared to be substantiated by various cases of natural obstruction of the duct, or by certain malformations of the part, where it was either defective, or did not convey its contents, in the ordinary manner, into the veins. The other set of arguments, which are more purely anatomical, were derived from the supposed fact that various parts of the body, which were evidently subject to the operation of absorption, were without lymphatics, and that this was likewise the case with large classes of animals, the general structure of which, as far as regards their growth and nutrition, was analogous to that of the mammalia. Admitting these data, it seemed to be a necessary conse quence that absorption must in these instances be performed by the veins, and hence it was inferred that in all classes of animals, and in all parts of the body, the veins co-operated with the lacteals and the lymphatics in the function of absorption.
The doctrine of venous absorption was first formally called in question, nearly at the same time,t by Wm. Hunter and by Monro se cundus,T who, as it would appear, to a certain extent, entered upon the investigation inde pendently of each other. The priority of dis covery in this, as in so many points connected with anatomy, was for a long time the subject of warm controversy. We may remark con cerning this question, that if the judgment of the present age should incline to ascribe to Hunter the original conception of the hypo thesis, it is also disposed to allow to Monro the merit of establishing his opinion by a skilful and laborious process of experiment and observation.
The method which these illustrious rivals adopted was, first, to repeat the experiments of their predecessors, when, by noticing with scrupulous accuracy all the circumstances con nected with them, they were able to demon strate, or at least to render it highly probable, that in all those cases where injections had passed between the absorbents and the veins, either rupture or extravasation had taken place, and that, when this was carefully guarded against, the supposed connexion between the two sets of vessels could not be demonstrated.* In those experiments, where the thoracic duct had been artificially obstructed, or in the cases where the same thing had occurred as the result of disease or malformation, they were enabled to detect some supplementary vessels or some indirect channel, by means of which the chyle had been conveyed to the veins.