224 BIOLOGY. [BooK n. 



and are animated by regular pulsations. Sixty beatings a j 

 minute have been counted in a frog. In these special points the 

 lymphatic wall, which elsewhere has nearly the same anatomical 

 texture as the veins, is clothed with a muscular layer thick and 

 striated. 



It is well known that in the mammifers and in man the lym- 

 phatic system is constituted by very fine capillary networks, 

 situated in the depth of the organs, in the various membranes, 

 and in the skin. From these networks proceed vessels which 

 throw themselves into each other, forming trunks larger and 

 larger, and less and less numerous. Finally all the system is 

 connected with the venous system by two canals. The smallest 

 of these canals receives the lymph from the right half of the 

 trunk and of the head ; it throws itself into the subclavial vein 

 on the same side. The other, larger, known under the name of 

 thoracic canal, receives the lymph from the rest of the body, and 

 has its conflux in the left subclavial vein. In man the lymphatic 

 network does not seem to have any other direct communications 

 with the sanguineous system. 



After every repast, the lymphatic vessels of the intestine, the 

 chyliferous vessels, swell and become lactescent. If we bind 

 the lymphatic canal we see it swelling below the ligature ; and if 

 we prick it we see the lymph spurt out. This alone suffices to 

 prove that the lymph pours itself into the blood ; and the arrange- 

 ment of the lymphatic valves also confirms this fact. But on its 

 passage the lymph encounters special organs, distentions, or closed 

 glands, from which it seems to obtain a supply of white globules. 

 These important organs are also subject to the law of gradual 

 differentiation. In fishes we find at distant intervals, on the 

 passage of some lymphatic vessels, simple enlargements, where 

 globules form in meshes of cellular tissue. 



In the reptiles there are already more prefect formative 

 organs ; they are the closed follicles or Peyer's glands, disseminated 

 in the depth of the intestinal mucous membrane. These follicles 

 are constituted by closed vesicles filled with nucleated cells; 



