The lymphatic glands are most developed in childhood, least so in old age, and are interme diate in this respect in adult life. They are not found in Amphibia and Fishes, and in Birds only in the cervical region : intricate plexuses of large lymphatic vessels occur fre quently in those animals which are destitute of lymphatic glands.
The colour of the lymphatic gland, depending apparently on the contents of its bloodvessels, is of a pale rose pink, resembling in this re spect the colour of the salivary glands or of the cineritious matter of the brain ; the exceptions to this observation will be found in the mesen teric glands while the chyle is passing through them, when they assume a whitish colour ; the lymphatic glands in the neighbourhood of the liver and gall-bladder have been observed to possess a slight yellow tinge, but this is tobe con sidered a post-mortem appearance. The black colour of the bronchial glands is remarkable and not easily accounted for ; the lymph passing from the lung to them being always perfectly transparent and colourless.
The lymphatic gland has a capsule of con densed cellular tissue, which surrounds it and firmly adheres to it, appearing to send cellular prolongations into its substance ; the outer sur face of this capsule is connected to the surround ing textures by a loose cellular tissue. The capsule appears to serve the purposes of convey ing the bloodvessels to the interior of the gland, of isolating it from the surrounding parts, and of preventing its over-distension by the lymph conveyed to it.
The bloodvessels of the lymphatic glands are large and distinct ; frequently more than one artery is traced to a gland ; the returning veins do not generally correspond either in direction or number with the arteries. The veins are much larger, but have appeared to me fewer in number than the arteries.
Nerves of considerable size pass to the lym phatic glands and can generally be traced through them, from which circumstance it has been doubted whether any filaments are left in the gland; but if acute sensibility to pain from undue pressure or from disease be admitted as dependent upon a proper supply of nerve, un doubtedly they possess it. The exact mode of arrangement of the bloodvessels in the interior of the gland is not well known. After a success ful injection of these vessels the gland assumes the same colour as the injection itself.
Our knowledge of the structure of the ab sorbent glands rests mainly upon the informa tion obtained by throwing injections of mercury or coloured wax into the lymphatic vessels. In
this mode of investigating their texture, the walls of the canals or cavities containing the injection, which appear, as in the kidney and testicle, to form the parenchyma of the organ, are com pressed, and when dry become transparent. the arrangement of the minute bloodvessels on the lining membrane of these canals has not been sufficiently investigated, and until this has been effected, our knowledge of the structure and function of the lymphatic gland must be considered very unsatisfactory, and as consist ing of little more than conjecture. The great point of controversy has been, whether the in jection thrown into the gland by the afferent lymphatic vessels was contained in cells or in convoluted vessels, which if decided would throw but little light upon the office performed by the gland—a desideratum in physiology of considerable importance, and without which we are left in the dark at the very threshold of our investigations with respect to the first changes effected in the lymph and chyle, in advance towards sanguification. On examining the glands thus distended with injections, the vasa inferentia are seen reaching the gland from various sources, and on their approach to it they may be observed to subdivide into ex tremely minute branches, which disappear by plunging into its substance : equally minute vessels may be observed emerging from its opposite side or surface, which soon unite to form the vasa efferentia of the gland ; the gland itself, which is intermediate in position between these vessels, when injected, presents a gra nular surface, and at first sight an observer would generally conclude that he was look ing upon minute cells filled by the injection ; in making a section also into the substance of the gland and allowing the mercury to escape, the appearance on a superficial in spection is still that of cells ; proceeding, however, with more attention to examine these supposed cells, especially after making a section as close to the surface of the gland as possible, by the aid of the microscope it will be evident that tubes closely set together and adherent to each other, have been laid open, passing in various directions, and in their interior many valvular constrictions and thread-like intersec tions may be seen ; in fact the gland appears to be entirely composed of a convoluted vessel, the sides of which as they come in contact me firmly held together by cellular membrane derived from the capsule.