The lacteals and lymphatics all terminate idthe thoracic duct,—a vessel of considerable size, that is situated along the back part of the thorax. It receives at its lower end the contents of all the absorbents, and pours them into the left subclavian, not far from the termination of the venous system in the right auricle of the heart. The thoracic duct, in its structure and functions, agrees generally with the other absorbent vessels; like them, it is thin, elastic, and contractile, and possesses numerous valves.
With respect to the use of the absorbent system, their office is literally expressed by their name; it consists in taking up certain substances, which they transport from one part of the body to another. The action of the lac teals is confined to the chyle, while the lymphatics receive a variety of substances that are presented to them. The thoracic duct is to be regarded merely as a receptacle, where the contents of the smaller vessels are lodged until the veins are able to receive them. So far we appear to proceed upon certain grounds ; but the office of the glands, which form so important a part of the absorbent system, is not so obvious : nor indeed can it be expected that we should understand this point, while we are still unac quainted with their structure. It is yet undecided whe ther the gland is merely a convolution of the absorbent vessel itself, or whether it consists of other vessels de rived from the artery, and only communicating with the absorbents. Upon the whole, the results of the most mi nute investigations seem to favour the opinion, that the gland is formed by a convolution of the absorbent system itself, and the office of these organs is rather to effect some change in the chyle or lymph, by the action of its components upon each other, than to add to them any ex traneous substance.
But although it is admitted as a general fact, that ab sorption is carried on by the lacteals and lymphatics, it has been a very warmly controverted point, whether this operation be exclusively performed by these vessels ? It was supposed, that the veins likewise possessed the pow er of absorption; and, in some parts of the body, and in some animals, they were conceived to be the principal, or even the sole agents in this process. This was the common opinion until the middle of the last century, and is the doctrine that was maintained both by Bocrhaave and by Haller. Since that time, however, the lymphatics
have been discovered in many organs where they were not then known to exist, and, in consequence, partly of the universal distribution of these vessels, and partly of direct experiments that were performed by Hunter, Mon t), and others, the doctrine of venous absorption was al most universally abandoned, and there were few facts in physiology which seemed to be more completely estab lished, than the exclusive absorbing power of the lacteals and lymphatics. The old opinion, however, has been of late brought forward by Al. Magendie, and supported by arguments and experiments which, it must be allowed, will render it necessary for us to pause, and carefully re view our former conclusions.
But whatever we may think upon this point, we know that the use of the lacteals and lymphatics is to absorb; and we are now to inquire, what is the respective office of each, or what kind of substances are they, each of them, destined to convey ? That of the lacteals is obviously con fined to the absorption of chyle ; and as this is the direct source whence the blood receives its supply, we are to re gard these vessels as the immediate organs of nutrition. The lymphatics may likewise occasionally contribute to the same end, but the nutrition of the body would not ap pear to be their ultimate object, because the substances which are taken up by them have been, for the most part, previously organized, and have already entered into the composition of the body, so that it would reduce their ac tion to a mere alternation of the processes of decomposi tion and reproduction. \Ve must, therefore, look for some other and more remote object in the lymphatics thar the supply of nutrition ; and we are led to suppose thaw their principal use is that of moulding and fashioning the body, and enabling it to acquire an increase of size with out deranging its form. When we reflect upon the man ner in which the body grows, we must be aware that it cannot be either by the distention of the parts already formed, or from the mere accretion of new particles to their surfaces, by an operation similar to crystallization ; for neither of these processes are applicable to organized bodies, where the change consists not in an ino ease of bulk only, but in a change of the whole internal structure, and a corresponding change in the external form.