AND CIRCULATION. 



113 



complete circuit, but emptying into various cavities which 

 interrupt their course. 



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

 brates, we find the vital fluid enclosed in an appropriate set 

 of vessels, by which it is successively conveyed throughout 

 the system to supply nutriment and secretions, and to the 

 respiratory organs, where it absorbs oxygen, or, in other 

 words, becomes oxygenated. 



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

 kinds: 1. The arteries, of a firm, elastic structure, which 

 may be distended or contracted, according to the volume of 

 their contents, and which convey the blood from the centre 

 towards the surface, distributing it to every point of the 

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

 structure, furnished within with valves, (Fig. 



82, v,) which aid in sustaining the column of 

 blood, only allowing it to flow from the peri- 

 phery towards the centre. The arteries con- 

 stantly subdivide into smaller and smaller 

 branches ; while the veins commence in minute 

 twigs, and are gathered into branches and larger 

 trunks, to unite finally into a few stems, near the 

 centre of circulation. 



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

 nected by a net-work of extremely 



delicate vessels, called capillary ves- 

 sels, (Fig. 83.) They pervade eveiy 

 portion of the body, so that almost 

 no point can be pricked without 

 drawing blood. Their office is to 

 distribute the nutritive fluid to the 

 organic cells, where all the important processes of nutrition 

 are performed, such as the alimentation and growth of all 

 organs and tissues, the elaboration of bile, milk, saliva, and 



10* 



Fig. 82. 



