176 CIRCULA'l'ORY SVSTEiM. 



muscles, and are more intimately and firmly compacted ; the 

 cellular tissue also, uniting them, is finer, denser, and less in 

 quantity. 



The heart is supplied with blood by the two coronary arteries; 

 the first branches given off from the aorta. Its veins pour their 

 blood into the coronary vein, by which it is returned into the 

 right auricle. Its nerves are derived from the cardiac plexus. 



OF THE BLOODVESSELS. 



There are two orders of bloodvessels — arteries and veins: 

 the former conduct the blood from the heart to all parts of the 

 body ; the latter return it therefrom back to the heart. 



ARTERIES. 



These vessels, in all their manifold ramifications, spring origi- 

 nally from two main trunks — the pulmonarj/ artery and the 

 aqrta : the former sends its branches to the lungs ; the latter to 

 all the other parts of the body. 



PULMONARY ARTERY. 



A vessel of larger caliber than the aorta. It takes its origin from 

 the postero-superior part of the right ventricle of the heart, winds 

 upwards to the root of the left lung, and there divides into right 

 and left pulmonary arteries ; which divisions immediately enter 

 the substance of their correspondent lungs, and therein ramify to 

 capillary minuteness, the branches regulating their course and 

 division by the ramification of the bronchial tubes. 



This trunk, together with its manifold branches, may be com- 

 pared (viewing them altogether) to a short, but straggling and 

 very branchy shrub or dwarf tree of luxuriant but extremely irre- 

 gular growth ; and their number and ramification may be pic- 

 tured to the mind, by remembering that no organized part of the 

 body is without few or many of them. 



