With respect to the actual nature of the con tents of the lymphatics there appears to be some uncertainty. We have the analysis of the fluid taken from the vessels of a dog by M. Chevreul,* from which it would appear that the lymph contains nearly the same in gredients with the blood, but diluted with a much larger proportion of water. We must, however, suppose that the fluid contained in the lymphatics will vary very considerably in its composition, according to the part of the body from which it is taken, or the condition of the same part at different times; yet we are scarcely able to detect an actual state of things which altogether corresponds with what we might have been led to expect would have been the case.-t- It may indeed be presumed that in the ordinary condition of the system, the process by which the parts of the body are absorbed is so very gradual, that the change in the chemical constitution of the lymphatic fluid is as inconspicuous as the change in the organs from which it is absorbed, and that it is only in morbid cases, where there is some extraordinary quantity of matter to be re moved, that we should expect to be able to detect it in the lymph. And this, to a certain extent,' agrees with the fact; for when the ab sorbents are called into action to remove col lections of pus, or when they become the vehicles of any poisonous or morbid body, the substance in question has been occasionally found in them.
The doctrine of the removal or absorption of all the parts of the body is rendered evident by a variety of cases, in which any particular organ or texture is broken down or removed, merely by cutting off the supply of fresh matter. It is upon this principle that we explain the removal of a part by pressure. If a muscle, or even a solid bone be exposed to constant pres sure, by which its nutritive arteries are ob structed, it will be gradually diminished in bulk, and at length completely abstracted. And this is frequently effected by the action of a body much softer than the substance which is removed, as, for instance, we observe a bone to be absorbed by the pulsation of a blood vessel, or the growth of a fleshy tumour.s But although we may venture to affirm that this moulding of the body, or rather of its in dividual parts, is effected by the lymphatics, either alone or in conjunction with the veins, there is considerable difficulty in forming a distinct conception of the mode in which they operate. The operation cannot, strictly speak ing, be mechanical, nor have we any evidence of the existence of a chemical solvent, by which the parts may be reduced to a liquid state, so as to fit them for entering into the mouths of the vessels. We may conceive of the source of supply being cut off by pressure or in other ways, but still we are at a loss to account for the mode in which the solids are either dissolved or broken down, so as to adapt them to the process of absorption. There is,
however, one principal or general fact in the animal economy, which will probably some what assist us in our inquiry, viz. that it appears to be essential to the well-being, or even to the existence of the corporeal frame, that all the materials of which it is composed should un dergo a constant change. It appears that these materials, after a certain length of time, expe rience some alteration in their nature, by which they are rendered unfit for the further perform ance of their functions as constituents of the living body. They are therefore removed and are replaced by fresh matter, this interchange being brought about in the gradual manner which was described above. Now this process implies a constant decomposition of the parts of the body, and as this decomposition is effected particle by particle, it may not be un reasonable to conjecture, that each particle, when it ceases to form an integral part of an organ, is left in a state proper for being taken up by the absorbents. But independent of any hypothetical views of this description, we may assume it as a probable conclusion, that the configuration and moulding of the body is the specific and appropriate office of the lymphatics, while its nutrition is effected more immediately by the lacteals.
With respect to the lymphatic glands we have seen above that their structure is involved in considerable obscurity, and we may remark, that their use is at least equally obscure. Among other opinions that have been entertained on the subject, some physiologists have supposed that the glands are proper secreting organs, which are destined for the purpose of preparing a peculiar substance that is mixed with the chyle and the lymph, or that they merely serve the mechanical purpose of mixing together more completely the constituents of the fluid that is contained in the vessels, and thus produce some change in its nature or. consistences There do not appear to be any arguments, either anatomical or physiological, by which this point can be decided ; but we may remark, that while the number and mode of distribution of these glands in the mammalia would seem to point them out as performing some important office in the animal economy, their rarity in birds and fishes proves that they are not essen tial to the existence of most of the functions of animal life, nor have we any mode of explaining the cause why they should be more necessary to the mammalia than to the other classes, which in many of their functions so nearly re semble them.