304 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 body 
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. Is 
As was intimated above there is a constant S a 
osmosis of fluid from the blood capillaries into — Fic. 310.—Scheme 
the surrounding tissues. This finally passes into °% oes eae ee 
the distal capillaries of the lymph system, while +,atrium; /, lymph ves- 
i Z : sels; Js, lymph sinus; v, 
in the walls of the alimentary canal there are, in ventricle; vs, venous 
addition, the results of the digestive processes ‘us 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. 
