CHAPTER XIY. 



LYMPHATIC CIRCULATION. 



THE lymphatic circulation does not seem to exist in the jn ver- 

 tebrates, unless we consider as lymph the colourless nutritive liquid 

 which travels along the rudimentary vessels in many of them. 

 Nevertheless certain persons have professed to recognise a 

 lymphatic circulation in the hirudinates, which are moreover 

 provided with a tolerably complete circulatory system. 1 



The lymphatic apparatus is assuredly an apparatus of per- 

 fectionment ; for it is entirely lacking in the amphioxus, and 

 does not appear in the vertebrated embryon till the late period 

 when the venous and arterial networks are already differen- 

 tiated. 2 The manner in which the lymphatic system com- 

 plicates itself by degrees in the veins of the vertebrates is 

 interesting ; for it suggests conjectures respecting the physio- 

 logical province of this system. 



In the inferior vertebrates, the lymphatic vessels are especially 

 represented by a kind of sheaths, surrounding the sanguineous 

 vessels, especially the arteries (Fig. 23). In fishes and the 

 batrachians we find this arrangement extending even to 

 the aorta. In all the vertebrates, furthermore, the fine lym- 

 phatic networks, the radical networks, a kind of lymphatic 

 capillaries, are joined to the sanguineous capillaries,- so that 

 these last form a part of their walls. In all the vertebrates, 



1 Leydig, Traiti tf Histologie de VHomme et dcs Animaux. 



2 Gegenbaur, loc. cit., p. 9. 



