OF THE BLOOD A]ST> CIECULATIOX. 



199 



movements of the animal, and partly by means of the incessant 

 vibrations of cilia, which overspread the interior. In most of 

 the mollusca and articulata, the blood, chyle, is also in imme- 

 diate contact with the viscera, water being mixed with it in the 

 mollusca ; the vessels, if there are any, forming a complete 

 circuit, but not emptying into various cavities which interrupt 

 their course. 



356. In animals of still higher organization, as the verte- 

 brata, we find the vital fluid inclosed in an appropriate set of 

 vessels, by which it is successively conveyed throughout the 

 vm, to supply nutriment and secretions, and to the respi- 

 ratory organs, where it absorbs oxygen, or, in other words, be- 

 comes oxygenated. 



357. The vessels in which the blood circulates are of two 

 kinds : 1 . The arteries, of a firm, elas- a 



tic structure, which may be distended, 

 or contracted, according to the volume 

 of their contents, and which convey 

 the blood from the centre towards the 

 periphery, distributing it to every point 

 of the body. 2. The veins, of a thin, 

 membranous structure, furnished with- 

 in with valves (fig. 216, v), which aid in 

 sustaining the column of blood, only 

 allowing it to flow from the periphery 

 towards the centre. The arteries con- 

 stantly subdivide into smaller and 

 smaller branches, while the veins com- 

 mencing in minute twigs, are gathered 

 into branches and larger vessels, to 

 unite finally into a few trunks near the centre of circulation. 



358. The extremities of the arteries and veins are con- 

 nected by a net- work of extremely delicate vessels, called capil- 

 lary vessels (figs. 224, 225) ; which pervade every portion of 

 the body, so that almost no point can be pricked without 

 wounding some of them. Their office is to distribute the nu- 

 tritive fluid to the organic cells, where all the important pro- 

 cesses of nutrition are performed, such as the alimentation and 

 <.vth of all organs and tissues, the elaboration of bile, milk, 

 saliva, and other important products derived from the b*lood, 

 the removal of effete particles, and the substitution of new 

 ones, and all those changes by which the bright blood of the ar- 



Fig. 216. Vein laid open, 

 to shew the valves, v, v. 



