334 COMPARATIVE ANATOMY 



vessels, as well as on the bulbus arteriosus and ventricle, lying 

 in the connective-tissue surrounding these structures. Numerous 

 independent lymphatic vessels may, however, also be present, 

 arising from a capillary network under the skin, and extending into 

 the intermuscalar septa ; the intestinal tract and the viscera are also 

 generally provided with definite lymph-vessels in the Amphibia 

 and Amniota. 



Contractile lymph-hearts may be present in connection with the 

 vessels. They occur in Fishes, but are much better known in 

 Amphibians, Reptiles, and Bird-embryos. Thus, in Urodeles, nume- 

 rous lymph-hearts are present under the skin along the sides of 

 the body and tail, at the junction of the dorsal and ventral body- 

 muscles ; in other Amphibians they are either confined to the poste- 

 rior end of the body (pelvic region), or, as in the Frog, are present 

 also between the transverse processes of the third and fourth 

 vertebrae. In Reptiles posterior lymph-hearts only are present, 

 and are situated at the boundary of the trunk and tail regions, 

 close to the transverse processes or ribs. Similar structures are not 

 known to be present in Mammals. 



Large lacunar lymph- sinuses are present under the skin of tail- 

 less Amphibia, and the skin is thus only loosely attached to the un- 

 derlying muscles. These subcutaneous lymph- sinuses are connected 

 with those of the peritoneal cavity. Amongst the latter, the sub- 

 vertebral lymph-sinus is of great importance in Fishes, Dipnoans, 

 and Amphibians ; it surrounds the aorta and is connected with the 

 (mesenteric) sinus lying amongst the viscera, into which the 

 lymphatic vessels of the intestine open. In Fishes and Dipnoans 

 there is also a large longitudinal lymphatic trunk lying within the 

 spinal canal. 



As already mentioned, the higher we pass in the animal series 

 the more commonly are lymphatic trunks with independent walls to 

 be met with. From Birds onwards a large longitudinal subverte- 

 bral trunk (the thoracic duct) is always present. In Mammals this 

 arises in the lumbar region, where it is usually dilated to form the 

 cisterna or receptaculum chyli ; it receives the lymph from the 

 posterior extremities, the pelvis, and the urinogenital organs, as 

 well as the lacteals, or lymphatics of the intestine. In Mammals it 

 communicates anteriorly with the left, and in Sauropsida with both 

 left and right precaval veins. The lymphatics of the head, neck, 

 and anterior extremities open into the same veins. 



The lymphatic vessels of Birds and Mammals are, like certain 

 of the veins, provided with valves, the arrangement of which allows 

 the lymph stream to pass in one direction only, i.e., towards the 

 veins. 



The lymph, as already mentioned (p. 299), consists of two 

 elements, a fluid (plasma) and cells (lymph-corpuscles, leucocytes) ; 

 and similar cells are present in the lymphoid or adenoid tissue which 

 occurs beneath the mucous membrane in various parts of the body 



