34 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



tend into all parts of the body, but of their development little is known. 

 Anastomoses occur between the vessels of the two sides of the bodv 



J 



and not infrequently the thoracic duct of one side shows more or less 

 degeneration, resulting in a lack of symmetry in the adult. 



Not enough is known of the distribution of the lymphatic trunks to 

 render broad generalizations possible, but it may be said that the sys- 

 tem is most extensively developed in the subcutaneous tissue, in the 

 corresponding envelopes (meninges) of the central nervous system, 

 in the intermuscular connective tissue, in the walls 

 of the alimentary canal, and, as a network, in close 

 connexion with the blood-vessels of the body. 



The lymph hearts are enlarged and contrac- 

 tile portions of the lymph vessels, provided with 

 valves to prevent backflow of the fluid (fig. 310). 

 Usually these contract by means of the intrinsic 

 muscles of the walls, but in some urodeles (Am- 

 blystoma) there is an unpaired lymph heart beneath 

 the truncus arteriosus which enlarges and con- 

 tracts with the systole and diastole of the blood 

 heart. 



As was intimated above there is a constant 

 osmosis of fluid from the blood capillaries into 

 the surrounding tissues. This finally passes into 



FIG. 310. Scheme 



the distal capillaries of the lymph system, while a, atrium; /, lymph ves- 



. ,, , ,, ,. . sels; Is, lymph sinus; v, 



in the walls of the alimentary canal there are, in ventricle; vs, venous 

 addition, the results of the digestive processes sinus of caudal vein. 

 added to the fluid in the lymph vessels. As this latter portion has a 

 milky appearance, due to the contained fat, it is called chyle and 

 the lymphatics which contain it are called lacteals and chyle 

 ducts. All of these additions to the contents of the lymph vessels 

 make a current in the larger lymph trunks, and finally the whole 

 of the lymph is returned to the veins by the several connexions already 

 mentioned. In addition to the propelling force of the lymph hearts and 

 the pressure due to absorption and osmosis, the lymph is also carried 

 along by the motions of the parts in which the vessels ramify, their 

 pressure being supplemented by the action of the valves. 



In those fishes which have been accurately studied the lymph system is well 

 developed and opens into the veins in the cardiac and caudal regions. The vessels 

 are especially developed in the tail, where (myxinoids, teleosts) lymph hearts occur. 



