. Elements of Comparative Anatomy. LYMPHATICS OF VERTEBRATA. 599 extremities. In the Eeptilia the subcutaneous lymphatic cavities are more varied and numerous, and the system is more intimately related to the arteries; the lymphatic vessels are sometimes wide spaces (Fig. 340), which surround the arteries and are traversed by trabecule; sometimes they form plexuses which accompany these vessels. When their trabecule are more largely developed, the lymphatic cavity is broken up into several anastomosing canals. The space which surrounds the aorta is broken up, in the Crocodilini and Chelonii, i
. Elements of Comparative Anatomy. LYMPHATICS OF VERTEBRATA. 599 extremities. In the Eeptilia the subcutaneous lymphatic cavities are more varied and numerous, and the system is more intimately related to the arteries; the lymphatic vessels are sometimes wide spaces (Fig. 340), which surround the arteries and are traversed by trabecule; sometimes they form plexuses which accompany these vessels. When their trabecule are more largely developed, the lymphatic cavity is broken up into several anastomosing canals. The space which surrounds the aorta is broken up, in the Crocodilini and Chelonii, into two trunks which sui'round the veins of the anterior extremity; and lymphatic vessels from the head, neck, and extremities open into them. The lymphatic trunks of Birds have the same characters, but, in them, both the large trunk in front of the aorta (thoracic duct), and the small vessels are more inde- pendent. As in the Eeptilia, the thoracic duct opens into the superior ven^ cavse (venae brachiocephalicfe). At the commence- ment of the tail the lymphatic system is also connected with the ischiac veins, or with the afferent renals, in which point they resemble the Amphibia and Eeptilia. In the Mammalia the walls of the lymphatic system are still more differentiated, although it often happens that in them also the sheath of the arteries bounds the course of part of the lymphatic current. Where they do not accompany the blood-vessels they form frequent anastomoses, or wide-meshed plexuses, and are dis- tinguished by valves, as are the same parts in Birds. The lymphatic vessels of the hinder extremities, as well as the chyle-ducts, unite into a chief trunk in the abdomen, which is rarely paired, and the origin of which is frequently distinguished by a considerable enlarge- ment (cisterna chyli). Thence they are continued into a thoracic duct, which opens into the commencement of the left brachio-cephalic vein; the trunks of the lym- phatics of the anterior parts of the body
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