CHAP, xiv.] 



LYMPHATIC CIRCULATION. 



223 



too, the lymphatic vessels sheathe the sanguineous vessels of 

 the nervous centres which they isolate thus from the nervous 

 substance. It is this intimate union of the sanguineous and 

 lymphatic networks which made many anatomists believe that 

 the brain had no lymphatic vessels. 1 



But the fine lymphatic canals do not seem to have always 

 walls of their own; in that case they must be a kind of 

 lacunae of the conjunctive tissue. 



FIG. 23 



Fragment of the aorta of a 

 tortoise (chelydra) sur- 

 ronnded by a large lym- 

 phatic cavity: a, aorta; 

 b, external wall of the 

 lymph.itic cavity removal 

 at b' ; trabecules fastening 

 the sanguineous vessel to 

 the walls of the lymphatic 

 cavity. 



FIG. ^4. 



a a, caudal sinus ; b, 

 transversal anasto- 

 mosis ; c, lateral 

 vessels ; d origin of 

 the caudal vein in 

 the silurus glanis. 



In the inferior vertebrates the lymphatic vessels communicate 

 with the veins by numerous branches. In the reptiles the 

 principal lymphatic vessels have just got so far as to have some 

 rare and insufficient valves. 



Numerous reptiles have lymphatic hearts, which we find, 

 moreover, in certain fishes, in amphibians, and even birds 

 (Fig. 24). These hearts are partial dilatations of the vessels, 

 1 Ch. Robin, loc. cit., pp. 214, 315. 



