(510 LYMPHATIC SYSTEM. 



various modes in which the blood-vessels and the nervous fila- 

 ments are distributed on their surface, no general or determi- 

 nate relation has been discovered between these structural con- 

 ditions and the peculiar properties of the secretions they pro- 

 duce. In their most complex conglomerate forms, the bilife- 

 rous, uriniferous, seminiferous, and other secerning tubuli, like 

 so many vascular systems, convey their fluid contents, elimi- 

 nated from the blood, always in one direction, from the san- 

 guiferous system, as the chyliferous and lymphatic systems 

 convey theirs to the mass of the blood. And although the 

 mode of action of these various emurictories of the circula- 

 tion, in producing chemical changes in the fluids they 

 transmit through their parietes, be, like the transmissive 

 powers of nervous filaments, yet unexplained, the most per- 

 fect regularity of system and unity of plan are not less 

 apparent in the development of secreting organs throughout 

 the classes of the animal kingdom and the embryos of indi- 

 viduals, than in the nervous or other important systems of 

 the animal economy. 



CHAPTER SIXTH. 



LYMPHATIC SYSTEM. 



IN the lowest tribes of animals, all the assimilative func- 

 tions are performed by the same simple digestive cavity 

 which receives the raw material from without ; but as we 

 ascend in the scale of organization, the functions become 

 more complicated in their result, and special organs are 

 appropriated to the several portions of the complex process 

 of nutrition. In the first condition of the vascular system in 

 the radiated classes of animals, the arteries are scarcely 

 distinguishable from the veins in structure or in function, 

 and to this simple plexus of ramified tubes, in which the 

 blood often takes a retrograde course, are confided the func- 

 tions of chylification, sanguification, circulation, and even 

 absorption. And throughout the highest of the inverte- 

 brated classes, the absorption of nutriment from the intes- 

 tine, and the absorption of the decayed materials from 



