42 THE CIRCULATION OF THE BLOOD 



large, macroscopical vessels to tiny, little, hair-like tubes that 

 cannot be seen with the naked eye. 



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 muscular 

 septum into two distinct compartments designated for conveni- 

 ence, 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 ventricle. There is 

 an opening between the right auricle and the right ventricle and 

 one between the left auricle and the left ventricle and each open- 

 ing 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 intercostal space. 



Covering and Lining. A serous sac, called the pericardium, 

 covers the heart. It hugs the muscle of the heart closely, com- 

 pletely 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 epithelial tissue. 



Structure. The muscle of the heart is striated, but contrary 

 to the usual rule, is involuntary in its action. The muscle 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. 



