336 COMPARATIVE ZOOLOGY OF CHORDATES 



blood is under pressure in the arteries and capillaries, owing to 

 the contraction of the smooth muscle surrounding the former 

 and of the Rouget-cells which compose the walls of the 

 latter. 



Lymphatics. — Attention may now be turned to the lym- 

 phatic system. In addition to the blood-vessels, the body 

 contains a system of vessels, channels, and spaces in which 

 lymph circulates, forming the lymphatic system. It is in 

 communication with the ccelomic cavity. Lymph is blood- 

 plasma and white corpuscles which exude from the capillaries 

 and bathe all the tissues of the body, supplying them with 

 nutritive products. From the tissues, the lymph (which may 

 thus be regarded as blood minus the red blood-corpuscles) is 

 gathered up into thin-walled channels, called the lymphatics. 

 These start from blind ends and eventually join the veins, in 

 particular the subclavians, the left of which receives the main 

 lymphatic trunk which is known as the thoracic duct. In the 

 amphibia the space between the skin and the muscles of the 

 body-wall is occupied by lymph, and in certain regions " lymph- 

 hearts " are present, with muscular walls, which propel the 

 lymph along. These lymph-hearts are lacking in mammals. 

 Lymphatic vessels are present in the wall of the intestine, and 

 are known as lacteals, for they absorb the fatty products of 

 digestion, and the milk-like emulsion which they contain gives 

 them a white appearance. Here and there along the lym- 

 phatics, lymph glands are formed. To these belong the 

 spleen (which first appears in the Selachii), the tonsils (derived 

 from the 2nd pair of visceral pouches), and Peyer's patches 

 along the intestine in the mammals. 



Literature 



Goodrich, E. S. Vertebrate Craniate, Cyclostomes and Fishes. Black, 

 London, 1909. 



Goodrich, E. S. On the Classification of the Reptilia. Proceedings of 

 the Royal Society, Ser. B, vol. 89. 1916. 



