LYMPHATIC SYSTEM. 293 



under the skin, and extending into the intermuscular septa 

 and the bases of the fins. The intestinal tract and the viscera 

 generally, of Sharks and Skates, are especially well provided with 

 lymphatic vessels. Elasmobranchs, moreover, possess a large 

 number of small lymph-hearts communicating with delicate 

 venous networks. Lymph-hearts are also to be met with in 

 Amphibians, 1 Reptiles, and Birds, but are fewer in number in the 

 two last-mentioned groups than in the first ; they are either con- 

 fined to the posterior end of the body (pelvic region), or, as in 

 the Frog, are present also between the transverse processes of the 

 third and fourth vertebrae. Their walls are capable of rhythmical 

 contraction, owing to the presence of muscular fibres. Similar 

 structures arc not known to be present in Mammals. 



Large lacunar lymph-sinuses are present under the skin of 

 tailless Amphibia, and the skin is thus only loosely attached to 

 the underlying muscles. These subcutaneous lymph-sinuses are 

 connected with those of the peritoneal cavity. Amongst the latter 

 the subvertebral lymph- sinus is of great- importance in 

 Fishes and Amphibia: 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 there is also 

 a large longitudinal lymphatic trunk lying within the spinal canal. 



The higher we get in the animal series the more commonly are 

 lymphatic trunks with independent walls to be met with; 

 thus from Birds onwards a large longitudinal subvertebral 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 lac teals or lymphatics of the intestines. In 

 Mammals it opens anteriorly into the left, and in Sauropsida into 

 both left and right brachiocephalic veins. The lymphatics of the 

 head, neck, and anterior extremities open into the same veins. 



The lymphatic vessels of Birds and Mammals are, like the 

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

 lymph- stream to pass in one direction only, and that a forward 

 one. 



The lymph, like the blood, consists of two elements, a fluid 

 (plasma) and cells (lymph-corpuscles, leucocytes). The latter 

 have been already mentioned and their important physiological 

 function indicated in the chapter on the alimentary canal. We 

 have seen that they migrate from the solitary follicles and Peyer's 

 patches through the mucosa into the lumen of the gut ; and the 

 same thing occurs with the leucocytes of the so-called tonsils. 

 These appear to be present only in Mammals, and have the form 



1 In Salamandra maculosa and Siredon piscifonnis, eight to twelve lymph-hearts 

 are present under the skin along the sides of the body and tail, at the junction of the 

 dorsal and ventral body-mnsclcs. 



