CHYLIFEROUS SYSTEM. 421 



tough, thin, fibrous layer, destitute of irritability, and the 

 strength and elasticity of which, allows of great distention 

 of the vessels without rupture of their delicate parietes. 



The chyliferous vessels of fishes not only appear to con- 

 sist of a single tunic, destitute of valves, and without those 

 conglomerate glands which they form in higher classes ; but 

 they are fewer also in number on the intestine, and between 

 the folds of the mesentery, and contain a more thin limpid 

 and colourless chyle, with less proportion of fibrin, arid 

 without oily globules. They are obvious in all kinds of 

 osseous and cartilaginous fishes, spreading on the mesentery, 

 and forming two layers on the coats of the intestine, com- 

 posing reticulate plexuses between the mucous and the 

 muscular tunics, and also more exteriorly between the 

 muscular and the peritoneal coats, where they continue 

 along the course of the mesentery. They appear to originate 

 in the highly vascular erectile villi, so commonly developed 

 from the mucous lining of the small intestines of vertebrated 

 animals, and which extend freely into the fluid periphery 

 of the chyme passing through the alimentary canal. The 

 chyliferous vessels of the intestine anastomose freely to form 

 numerous loops and plexuses in their course, and constitute 

 by their union larger and larger trunks, which pass on between 

 the folds of the mesentery to terminate in one or two dilat- 

 ed reservoirs or receptacula chyli. The only trace of the 

 mesenteric glands, (found on these vessels in many higher 

 animals) are seen in the tortuous ramifications and compli- 

 cated anastomosing plexuses which they often form in fishes, 

 in their course towards the general receptacle of the chyle ; 

 and indeed this the true constitution, though on a simpler 

 scale, of the so-named mesenteric glands of the highest 

 animals and man, which have never the structure or excre- 

 tory ducts of true glands. Two thoracic ducts proceed forwards 

 from the recepiaculum, and form also frequent unions by their 

 astomoses, as they proceed forward along the sides of the aor- 

 ta, to open into the branches of the superior and inferior cavte 

 or the jugular veins. These vessels, like the lymphatics, appear 

 to have communications with the veins, which accompany 

 them, and semi-lunar valvular folds are developed at the 

 orifices by which they communicate ; and although, by in- 

 jection, and by inflating air upon the inner surface of the 



