ABSORBENT SYSTEM. 83 



conspire to ensure perfect comminution of the food, retardation of 

 its passage, and a due absorption of its nutritious particles. 



CHAPTER XII. 



ABSORBENT SYSTEM. 



Although Monro, Poli, Sheldon, and Carus, have described chy- 

 liferous vessels in many of the invertebrated classes, their existence 

 in these animals has not yet been satisfactorily proved, and it is more 

 than probable that their function is performed by the veins, the white 

 blood of which resembles the chyliferous fluid of the vertebrata. 

 The chyle varies much in its composition and properties in the 

 various tribes of vertebrated animals, and even in the same animal 

 according to the sort of food it lives on, being of a pinkish tint, with 

 abundance of crassamentum in the different animals which subsist 

 on nutritious animal food, while it is limpid and pale, with a great 

 proportion of serum in the inferior animals. The chyliferous, like 

 the other systems of the body, presents different grades of develop- 

 ment in the different vertebrated classes. Thus, in fishes the 

 vessels seem to consist of a single tunic, destitute of valves, and 

 without conglobate glands ; they form two strata of vessels between 

 the coats of the intestine, and carry a limpid fluid to the recepta- 

 culum chyli, from which one or two thoracic ducts lead to the 

 jugular veins or other branches of the cavee. The lymphatics and 

 lacteals communicate frequently with one another, and with the 

 neighbouring veins, and when injected they present a beaded ap- 

 pearance owing to the presence of rudimentary valves. 



The chyliferous system in the amphibia is the same as in fish, 

 but in the reptilia it presents a higher degree of formation in the 

 existence of valves, and the milkish appearance of the chyle, the 

 place of glands is still supplied by the convoluted condition of the 

 vessels as in fishes ; here also, two or more thoracic ducts, fre- 

 quently communicating, pass to the jugular or subclavian veins, or 

 the angle between them, previously receiving the lymphatics from 

 the head, neck, and arms. In the tortoise, the anastomoses of the 

 thoracic ducts nearly conceal the trunk of the aorta. This system 

 presents a somewhat higher grade of development in birds, both 

 sets cf vessels being more numerous and distinct, the valves are 

 more abundant, but yet admit of the passage of fluids from trunks 

 to branches. Glands appear now for the first time, in connection 

 with the lymphatics, but not with the lacteals. Two thoracic ducts, 

 having but few anastomoses, terminate by several openings at the 

 junction of the jugular and subclavian veins. 



In the mammalia the absorbent system is better developed and 



