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 (of the 

 head and anterior extremities), and of the wall 

 of the thorax, open into, and on either side of 

 the same vein. 



The lymphatic trunks are generally widened 

 out near their opening into the veins, and the 

 wall of these enlargements is distinguished by 

 its muscular investment ; it executes rhythmic 

 contractions. These arrangements are known as 

 lymphatic hearts. They have been occa- 

 sionally observed on the caudal sinus of Fishes, 

 but they are more accurately known in the Ajn- 

 phibia (RanidEe) and Eeptilia (Chelonii) ; in the 

 former they are found on both the anterior and 

 posterior openings into the veins, but in the 

 urodelous Amphibia and in the Eeptilia only the 

 posterior lymphatic hearts have been made out. 

 found also in the Eatitge (Struthio, Casuarius), and some Natatores, 

 but in other Birds they have no muscular investment, and form mere 



Fig. 341. Caudal 

 sinus. a a, Anas- 

 tomosing transverse 

 trunk. h Lateral 

 vessels c, and origin 

 of the caudal vein 

 d. Of S i 1 u r u s 

 glanis (after Hyrtl). 



These latter are 



