94 THE HORSB. 



tained being fastened to the breast-bone (sternum). The sac in which 

 the heart is contained is called the pericardium (peri=around, cardium 

 =the heart). This is a membrane of a fibrous, dense nature, which is 

 lined with a delicate serous membrane, of which there are two layers, 

 the inner one closely fastened to the heart, the outer to the fibrous sac. 

 There is a space between them in which is found a small amount of 

 serum, which is secreted by this serous membrane for the purpose of 

 lubricating the surfaces, which rub against each other every heart beat. 



How Divided. The heart is divided into four cavities, having two 

 on the right side which contain the venous (blue) blood, and two on the 

 left which contain the arterial (red) blood. It is so divided that the 

 cavities on the right side have no communication with those on the left, 

 but the two cavities on either side are connected by valves. The upper 

 cavities are called auricles from their supposed resemblance to a dog's 

 ear. The one on the upper right side is called the right auricle, the one 

 on the left, the left auricle. The cavities at the lower part of the heart are 

 called ventricles, taking the names right ventricle and left ventricle accord- 

 ing to location. 



Action of Heart. The right side of the heart receives the dark 

 blue blood from the vena cava. It is received into the right auricle, 

 where it passes through the tricuspid valve into the right ventricle. It 

 is there forced out into the lungs through the pulmonary artery. The 

 blood here gives up its carbon-dioxide, takes on oxygen, and re- 

 turns to the left auricle, through the pulmonary vein. The blood, now 

 of a bright scarlet hue, passes through the bicuspid valves into the left 

 ventricle, from which it is forced out through the aorta into the arteries, 

 by which it is distributed over all parts of the body. 



Movements of the Heart. When the heart closes to force out 

 the blood, the movement is called systole, and when expanding, 

 DIASTOLE, There is a pause following such expansion. Hence the 

 heart takes a rest after each beat. This amounts to several hours per 

 day. 



Functions of Blood-vessels. The blood-vessels carry blood to 

 and from the various parts of the body. They are divided into three 

 classes: THE arteries and arterioles, carrying blood from the heart; 

 the capillaries, bring the blood into close relation with the tissues; 

 and the veins, carrying the blood back to the heart. 



Arteries. These are hollow tubes carrying blood to the capillaries, 

 which are situated in all parts of the body. The arteries contain no 



