ABSORPTION. 317 



course of the veins, till they finally discharge their 

 contents 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, 

 378 T*'4M which, when the vessel is distended with 

 lymph, give it a resemblance to a string 

 of beads, Fig. 378. 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. 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, in- 

 deed, possess lymphatics, and absorption must con- 

 sequently be performed by the veins, when these 

 latter vessels exist. The addition of the system of 

 lymphatic vessels, as auxiliaries to the veins, may 

 therefore be regarded as a refinement in organiza- 

 tion, peculiar to the higher classes of animals.* 

 Muller has lately discovered that the frog, and 



* Folimann, who has made extensive researches on the absorbent 

 vessels throughout all the classes of vertebrated animals, has found 

 that they terminate extensively in the veins. See his work, entitled 

 " Anatomische Untersnchungen uber die Verbindunp^ der Saugadern 

 mit den Venen." 



