CIRCULATION. 133 



CHAPTER VIII. 



CIRCULATION. 



I WILL commence this subject with a description of the heart and 

 blood-vessels, and then describe the circulation. 



In all those animals which most resemble man (anatomically), 

 such as the monkey, the dog, horse, ox, &c., the heart is- placed 

 between the two lungs, in the cavity of the chest, which anatomists 

 call the thorax. 



The general form of the heart is that of an inverted cone, the 

 apex down, and a little to the left. The size of the heart is 

 very nearly that of the fist of the individual to whom it belongs. 



This organ is enveloped in a double membranous sac, called 

 pericardium, and is suspended in the pericardium by the vessels 

 which arise from its superior and enlarged extremity ; but it does 

 not adhere at any other point of its surface to the neighboring 

 parts. 



The substance of the heart is almost entirely fleshy ; it is a hol- 

 low muscle, the cavity of which communicates with the arteries 

 and veins. 



In man and all the mammalia, as well as birds, it has four dis- 

 tinct cavities. A thick, vertical partition divides it into two halves, 

 each one forming two cavities, one above the other ; a ventricle, and 

 an auricle. 



The two ventricles occupy the inferior part of the heart, and do 

 not communicate with each other, but each one opens into the 

 auricle above it. 



The heart, then, may be described as a hollow muscle, which, in 

 birds and mammalia, as in man, is divided into four distinct cham- 

 bers, as shown in the following figure. This division is effected 



