CIBCULATION OF THE BLOOD, 



47 



body. The arteries, proceeding from the heart, divide and 



subdivide in their course as they proceed, until they become 



exceedingly small : the veins follow a different course ; they 



commence by extremely fine roots, and collect into large 



branches and trunks as they proceed towards the heart. In 



the arteries, the motion of the blood is from the heart ; and 



in the veins, towards the heart. The arteries terminate, and 



the veins commence, in a fine network of vessels formed 



by both; the vessels composing this 



network are called capillary. The 



heart being thus placed between the 



termination of the veins and the 



commencement of the arteries, the 



movement of the blood in man and 



higher animals is a complete circle, 



the blood always returning to the 



point from which it started; hence 



it has been called the circulation of 



the blood, 



In all animals in which the respira- 

 tion is performed by a special organ, 

 the blood is sent to it from the heart, 

 and retires from it by a special system 

 of canals ; the circulation thus esta- 

 blished is called the respiratory or the 

 lesser circulation, whilst that through 

 the body is called the greater. We 

 shall consider, first, the circulation in 

 man, and this will serve as a standard 

 of comparison with others. 



Fig. 29. Capillary Ves- 

 sels in the Foot of the 

 Frog.* 



Description of the Circulation of the Blood in the Higher 

 Animals. 



92. TJie Heart. In man, and all animals similarly or- 

 ganized, the heart is lodged between the lungs and in the cavity 

 of the chest called by anatomists the thorax (Figs. 4 and 49) ; 

 its lower extremity is directed somewhat obliquely towards 

 the left side and forwards ; and its upper extremity, from 

 which spring the great vessels, is fixed to the neighbouring 

 parts, nearly in the mesial plane of the body. Throughout 



* a, arteries, v, veins, 

 dilating fluids. 



The arrows point out the direction of the cir- 



