The principal lymphatics in any part of the body may be said, taken collectively, to equal the capacity of the arteries or veins of the same part ; thus, in the inguinal region the sum of the diameters of the lymphatic vessels may equal the diameter of the main channel by which the venous blood is returned from the lower extremity; but by this simple subdivision of the outlet for the lymph into numerous branches, that almost universal, and, in its effects, wonderful power, by which the nutrient fluid throughout the vegetable creation is car ried from the lowest fibril of the root to the highest living point in vegetable existence, is made available in the progression of the lymph in animals towards the centres of the system. This disposition of the lymphatic vessels throughout their course necessitates a greater uniformity in point of size, than we find to hold good with the artery or vein, and indeed constitutes their chief peculiarity in distribution when com pared with the other divisions of the vascular system. The arborescent appearance, except on the surface of the liver and spleen, is scarcely to be met with in the lymphatics ; they almost always form a net-work of vessels, the meshes of which vary both in form and size in the different organs and in different parts of the body ; as a general rule, when the vessels have a short course to run, the spaces they enclose are small and more nearly equi lateral; but when the contrary is the case, as occurs in the extremities, the meshes are very large and much elongated, so that the vessels run nearly parallel with each other, and the net-work arrangement is scarcely perceptible; there is, however, still less appearance of arbor escence.
In this respect the lymphatics may be said to resemble more the capillary bloodvessels, which in the web of the frog's foot, or in the vesicular lungs of the salamander, toad, or frog, are so plainly seen to form a net-work of nearly equal-sized vessels, and, indeed, to cease to he capillaries when they become arborescent.
Another peculiarity in the disposition of the lymphatic vessels occurs at their approach to a conglobate gland, through which their contents are to be conveyed. The vessels leading to a gland which are termed the vasa inferentia or afferentia of the gland, vary in number, being seldom less than two, and rarely amounting to more than five or six. They maintain their ordi nary size and appearance to within a quarter of an inch of the gland, where they suddenly branch out, artery-like, into several exceedingly minute vessels which plunge into the gland, thus con veying the lymph, in a minutely divided state through this organ to emerge again from it by a converse arrangement of equally small vessels, which at a quarter of an inch from the gland, are collected like so many small veins into one or more trunks, called the vasa cfferentia of the gland ; not unfrequently there is but one of these vessels passing from a gland, and rarely more than two or three; they are, however, generally larger than the vasa in ferentia, and often double theirsize. ( Fig. 52.) A si milar arrange ment in the bloodvessels before entering or passing from their appropriate glandular organs may be noticed in the spleen and kidney, but the only instance in which a bloodvessel col lecting its contents from branches assumes the opposite function of distributing them into narrow streams occurs in the vena porta, where the blood is to be passed through the liver to be subjected to its action. The
same object, it. is true, is effected at the heart with the blood of the venm cam, together with the lymph and chyle, when conveyed in capil lary streams through the lungs to be converted into arterial blood; the right side of the heart, however, here intervenes between the collect ing vessels and those which have to redistribute the blood ; the latter also are called arteries though they convey the same venous blood to the lungs which the former vessels brought to the heart.
The vasa inferentia are by most authors des cribed as entering that edge of the gland which is farthest removed from the trunks of the system, and the vasa efferentia that nearest to them. This I find not to be the case ; the vessels usually plunge into and emerge from the broadest surfaces of the gland ; sometimes it is the deeper surface, sometimes the more superficial, and frequently both. The vasa iuferentia may enter one surface, and the vasa efferentia pass from the same or the opposite surface of the gland. The vasa efferentia, as they proceed onward, become the vasa infe rentia of succeeding glands ; thus the lymph is often made to traverse several glands before it is received by the trunks of the system. This is so much the case in the neighbourhood of the thoracic duct, especially in the pelvic and abdominal cavities, that the lymphatic system assumes altogether a different aspect; the net work appearance, the uniformity in point of size, are lost sight of in the numerous large short vasa efferentia and inferentia, intervening be tween the closely set glands. The appropriate lymphatic vessels of the viscera and walls of these cavities, nevertheless maintain the ordi nary disposition, the apparent irregularity de pending upon the circumstance, that the large lymphatics of the lower extremities, are inter rupted by numerous glands in their passage to the thoracic duct.
The lymphatic vessels are distributed through out the body on two planes, one superficial, the other deeply seated. The vessels of the two planes where they approach each other commu nicate freely. A similar arrangement takes place partially in the venous system; and it is interest ing to remark that where this occurs, the veins, like the lymphatics, are armed with valves. We cannot fail to recognize here a double pro vision to facilitate the progress of the contents of a vessel towards their proper destination ; while the valves prevent effectually any retro grade movement, the double plane of vessels, by increasing the number of channels, lessens the liability to arrest from the various causes of obstruction. The superficial lymphatics accompany more or less the superficial veins where these occur, but in other parts of the body they assume various appearances peculiar to each viscus or organ ; the superficial lympha tics, of the liver, spleen, kidney, and lungs, for instance, differ materially from each other in arrangement and appearance. The deep-seated lymphatics every where follow the course of the large bloodvessels. They are fewer in number and perhaps rather larger than the superficial. The superficial and deep lymphatics commu nicate with each other in the lymphatic glands as well as in different parts of their course.