OF MAMMALIA. 233 



The circulatory system has its starting-point in a highly mus- 

 cular, fourfold sac, the heart, (h,) which lies about in the median 

 line of the chest, below the gullet (g) and close to the breast- 

 bone. The principal vessels connected with it are the arteries 

 which carry the blood to the head, lungs, and posterior regions 

 of the body, and the veins in which the return currents bring 

 back that fluid to the heart. The course of the blood in com- 

 pleting its circuit is rather complicated, if we follow it in all its 

 details, but the essential lines of travel are quite simple in their 

 connections. Starting from one of the chambers of the heart, 

 which is called the left ventricle, the blood passes, through the 

 great artery, (ao,) a short distance forward, and then, as it turns 

 upward, a smaller current diverges toward the head in what is 

 called the carotid artery, (ao 1 ,) whilst the main current (ao) con- 

 tinues up to the lower face of the spine and follows its line 

 backwards in what is known at this part of its course as the 

 abdominal artery. From the latter numerous vessels are given 

 off to the various visceral organs, and from the carotid (ao 1 ) the 

 organs in the neck and head are supplied with branches. The 

 return currents start in the minute, branching, capillary vessels 

 which are continuous with the equally minute, branching ter- 

 minations of the arteries. The capillary return vessels, veins, 

 gradually unite in fewer and larger vessels, which finally coalesce 

 in one large vein in front, coming from the head, and in another 

 great vein behind, (vc,) coming from the abdomen. Each vein, 

 the vena cava superior, and the vena cava inferior (vc), empties 

 its contents separately into the right auricle of the heart, and thus 

 one branch of the circuit is completed. The other branch of the 

 circulation is devoted purely to the aeration of the blood. The 

 venous blood passes from the chamber in which it was received, 

 i. e. the right auricle, into the right ventricle, and from thence it is 

 thrown through the pulmonic artery (la,) into the minute branch- 

 ing vessels of the lungs (I). After being aerated there, the blood 

 flows through the capillary veinlets into the great recurrent 

 vessels, the pulmonary veins, from each lung, and thence enters 

 the left auricle, and finally completes its tour by passing into 



