42 THE CIRCULATION OF THE BLOOD 



The central organ of the circulatory system is the heart. 

 From this lead off the arteries, these in turn connect with the 

 capillaries, and these with the veins, which lead back to the 

 heart. 



I. THE HEART. 



The heart is a hollow, muscular organ divided by a mus- 

 cular septum into two distinct compartments designated for 

 convenience, the right and left heart. The right side, and 

 similarly the left, is divided by a muscular septum into two 

 chambers, the upper called the auricle and the lower the ven- 

 tricle. There is an opening between the right auricle and 

 the right ventricle and one between the left auricle and the 

 left ventricle and each opening is guarded and can be closed 

 by a thin membranous flap called a valve. 



Situation. The heart is located in the thoracic cavity be- 

 hind the sternum. It is placed in a diagonal position and its 

 base is in the middle line and looks backward, upward, and 

 to the right. Its apex is three inches to the left of the median 

 line, a half inch internal to the nipple, and in the fifth inter- 

 costal space. 



Covering and Lining. A serous sac, called the pericar- 

 dium, covers the heart. It hugs the muscle of the heart 

 closely, completely enveloping the organ, then turns back on 

 itself leaving a space between the outer layer and the layer 

 next to the muscle. In this space is a fluid which acts as a 

 lubricant. 



The heart is lined by a membrane called the endocardium, 

 which is composed of endothelial tissue. 



Structure. The muscle of the heart is striated, but con- 

 trary to the usual rule, is involuntary in its action. The mus- 

 cle fibers run circularly, obliquely, and some in the form of 

 the figure eight, thus giving the power to contract and pump 

 the blood on into the circulation. 



