352 THE VITAL FUNCTIONS. 



intestinal cavity : they are found in all the 

 classes of vertebrated animals, and pervade 

 extensively every part of the body. Exceed- 

 ingly minute at their origin, they unite toge- 

 ther as they proceed, forming larger and 

 larger trunks, generally following the course of 

 the veins, till they finally discharge their con- 

 tents either into the thoracic duct, or into some 

 of the large veins in the vicinity of the heart. 

 Throughout their whole course they are, like the 

 lacteals, provided with numerous 

 valves, which, when the vessel is dis- 

 tended with lymph, give it a resem- 

 blance to a string of beads. Fig. 378.* 

 In the lower animals it appears that 

 the veins are occasionally endowed 

 with a power of absorption, similar 

 to that possessed by the lymphatics. None of 

 the invertebrata, indeed, possess lymphatics, and 

 absorption must consequently be performed by 

 the veins, Mhen these latter vessels exist. The 

 addition of the system of lymphatic vessels, as 



* In warm-blooded animals, the lymphatics are made to 

 traverse, in some part of their course, certain bodies of a 

 compact structure, resembling glands, and termed accordingly, 

 the lymphatic glands. One of these is represented in Fig. 378. 

 They correspond in structure, and probably also in their functions, 

 to the mesenteric glands, through which, in the mammalia, the 

 lacteals pass, before reaching the thoracic duct. It is chiefly in 

 the mammalia, indeed, that these glands are met with, for they 

 are rare among birds, and still more so among fishes and 

 reptiles. 



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