CHAPTER II 

 THE HEART 



The blood in the body has to be kept in constant motion, so that 

 the tissues which are depending upon it for their vitality may 

 be continuously supplied, and also in order that the impure fluid 

 resulting from the changes in the tissues may be rapidly and 

 effectually conveyed to those organs where its purification is 

 carried out. 



The heart is the organ which pumps the blood over the body, 

 not only distributing it to the tissues, but forcing it on from 

 these back to the heart again, to be prepared for redistribution. 

 It may be described as a hollow muscle divided into two com- 

 partments, usually known as right and left, but in quadrupeds 

 really anterior and posterior, each compartment being divided 

 into an upper half or auricle, and a lower or ventricle. Opening 

 into the auricles are large veins which convey the blood back to 

 the heart, while from the ventricles other vessels, arteries, take 

 their origin for the conveyance of blood from the heart. The 

 auricles and ventricles are separated by a valvular arrangement, 

 and the two sides of the heart are separated by a muscular 

 partition (Fig. 10). 



So far the general arrangement of both right and left sides is 

 much the same, each having to receive and then to get rid of a 

 certain quantity of blood sent into it. But the blood sent into 

 the right side of the heart is very different from that received 

 by the left, and with this difference we must for a moment deal. 

 The whole of the impure or venous blood in the body is brought 

 into the right side of the heart for the purpose of being distributed 

 to the lungs, where it is purified ; into the left heart this arterial 

 or purified blood is brought back from the lungs for distribution 

 to the body. The passage of the impure or venous blood from 

 the right side of the heart through the lungs to the left side is 

 known as the PiilmGwe- circulation ; that of the blood, thus 

 purified, through the body and back to the right side of the 

 heart is called the Systemic circulation (Fig. n). 



