420 CHYLIFEROUS SYSTEM. 



the red blood-vessels still more obvious in the mammiferous 

 class. 



The chyle contained in these vessels in the vertebrat- 

 ed classes, and derived from the digested chyme of their 

 alimentary canal, much resembles the white blood of the 

 lower divisions of the animal kingdonij and varies in its 

 composition and properties in the different tribes of animals, 

 and in the same animal according to the kind of food on 

 which it subsists, being most allied to red blood in the 

 reddish colour, and the abundance of its fibrinous crassamen- 

 tum in the highest animals, and those which subsist on the 

 most nutritious animal food, and most remote from that 

 condition in its pale and limpid character, and the 'great 

 proportion of its thin serum in the lowest fishes, and the 

 most impoverished animals. The light floating white co- 

 loured fatty globules, seen already formed in the chyme, 

 appear also to have the same relations to the chyle as to the 

 circulating mass of blood, in the different tribes of animals 

 and in the different conditions of their food. The elements 

 of this fluid are intimately mingled in passing through the 

 chyliferous vessels ; but their motions are not aided by pul- 

 sating ventricular sacs, like those developed on the lympha- 

 tics in situations where their fluids are less affected by the 

 movements of the surrounding parts. 



The same grades of development which are perceptible in the 

 properties and constitution of the chyle, are seen also in the 

 structure, forms and number of the vessels which convey it in 

 the different vertebrated classes, being fewer in number, des- 

 titute of internal valves, and apparently composed of a single 

 tunic in fishes, while in the class of mammiferous animals, 

 their numbers exceed all calculation, their valvular structure 

 is most universal and complete, and their two component tun- 

 cis are easily separable from each other. The inner coat is 

 a thin, smooth, serous membrane, which, by extending 

 more or less in free folds into the tubular cavity of the ves- 

 sels, produces the crescentic or semilunar valves, so nume- 

 rous and important in the higher animals, in directing the 

 motions of the contained fluid constantly towards the recep- 

 taculum chyli or the thoracic duct, these motions being de- 

 rived chiefly from the vis a tergo of the newly absorbed chyle, 

 and the incessant movements of the blood-vessels and the 

 surrounding viscera of organic life. The exterior tunic is a 



