LYMPHATIC SYSTEM 433 



In Urodeles, the single thoracic duct bifurcates near the heart, 

 thus indicating its primarily paired character ; it receives lymph- 

 vessels from the head, and each branch opens into the corres- 

 ponding subclavian vein. The arrangement of the lymphatics in 

 Urodeles, and still more in Anurans, becomes considerably 

 modified owing to the presence of lymph-hearts and of large 

 sinuses under the skin which communicate with those of the 

 peritoneal cavity (e.g. sub vertebral, perioesophageal) : the cephalic 

 and thoracic ducts disappear, and the only trunks which remain 

 are those connecting the lymph-hearts with the sinuses. Valves 

 occur in the lymphatics where they communicate with the lymph- 

 hearts, sinuses, and veins. 



In Reptiles there are two cephalic ducts which open into the 

 subclavian vein along with the thoracic duct, which is either 

 paired (Snakes, Chelonians, Crocodiles), or single, bifurcating 

 anteriorly. 



Lymphatic vessels are less developed in Birds than in 

 Mammals : two thoracic ducts arise in the region between the 

 thyroid gland and coeliac artery, having close relations to the 

 aorta and precavals, with the latter of which they communicate at 

 various points: their connection with the longitudinal channels 

 along the posterior part of the aorta takes place later (Fig. 332). 

 As in Reptiles, valves are not abundant, and occur in only a few 

 of the vessels. 



In Mammals both thoracic ducts may persist, or that of one side 

 may disappear. Thus in Man the duct is unpaired and usually 

 arises from a sinus (cisterna or receptaculum chyli) in the lumbar 

 region : it receives the lymphatics from the legs and pelvic organs 

 as well as those from the intestine (lacteals) and opens anteriorly 

 into the left brachiocephalic vein, with which also communicate 

 lymph-vessels from the head, neck, and right side of the thorax. 

 The lymphatics, like the veins, are provided with numerous valves 

 in Mammals. 



Rhythmically contractile lymph-hearts (of which little is known 

 in Fishes except that two are present, e.g., in the caudal region in 

 Silurus and one in the Eel), occur in Amphibians, Reptiles, and 

 embryos of Birds (Fig. 332): they are surrounded by striped muscle. 

 In Urodeles there are as many as 14-20 on either side of the trunk 

 and tail under the skin at the junction of the dorsal and ventral 

 trunk-muscles, 1 while in Anura the number is reduced to two 

 pairs with numerous valves, the anterior of which is situated 

 between the transverse processes of the third and fourth vertebras 

 and the posterior between the urostyle and pelvis. In Reptiles 

 posterior lymph-hearts only are present, situated at the junction 

 of trunk and tail on the transverse processes or ribs. In Bird 

 embryos they occur at the boundary between the sacral and coccy- 



1 In the walls of thetruncus arteriosus of Urodeles a lymph-sinus is included, 

 which has been described as the "central lymph- heart." 



F F 



