138 ANATOMY AND PHYSIOLOGY. 



fourth, the capillary (hair-like) vessels, which unite the termination 

 of the arteries with the commencement of the veins. 



Every one knows the appearance of the heart an ox's or a 

 sheep's, for example. When cut up, it is found to consist of four 

 cavities, two on the right side that communicate with each other, 

 but not directly with those opposite. The diagram or figure affords 

 an exact representation of the human heart, with the circulation of 

 the blood to and from the lungs on both sides. 



To make the course pursued by the blood more plain, we take 

 asunder the two sides or chambers of the heart, and represent them 

 as separate from, and opposite to, each other, as seen in the figure 

 adjoining. 



In this ideal plan of the circulation in the Mammalia, the arteries 

 and veins are supposed to be thrown into continuous chains, with 

 the capillaries as their connecting links. In describing this plan, 

 we shall commence with the veins. It will be seen that they 

 gradually unite, until those coming from above and those from below 

 form two large vessels (vence cava), that empty themselves into the 

 upper cavity of the heart on the right side, called the right auricle. 

 From the contraction or drawing together of this auricle, the blood 

 easily passes downwards into the next cavity, called the right ven- 

 tricle j and this, which is still more powerful, also in its turn con- 

 tracting upon the blood, sends it through the vessel (p a), called the 

 pulmonary artery. It is plain, however, that, if nothing hindered 

 it, the blood could as easily go back to the right auricle, as forwards 

 into the pulmonary artery ; but this is effectually prevented by a 

 valve that is placed between the right auricle and the right ventricle, 

 and which allows the blood to enter, but prevents it going back. 

 Another valve, which acts in a similar manner, is placed at the 

 mouth of the pulmonary artery, so that the blood, by the successive 

 contractions of the ventricle, is forced to go forward into the lungs. 

 Here, as will afterwards be explained, it becomes purified, as is 

 shown by its red color, and is sent by the pulmonary veins ( p v) to 

 the left auricle of the heart, to pass, as on the other side, into the 

 left ventricle. This last is the most powerful of all the parts de- 

 scribed, as it is required to propel the blood into the artery called 

 the aorta (o a), and from it into the whole of the body. Valves are 

 placed on the left side, between the auricle and ventricle, and at 

 the mouth of the aorta, which have a similar action and appearance 

 to those on the right side. Those between the auricles and ven- 

 tricles are called cuspid, that is, pointed valves ; on the right side, 



