THE CIRCULATION. 221 



from the heart to the various organs and parts of the body the 

 arteries and the vessels returning the impoverished blood to the 

 right side of the heart the veins. Connecting the smallest 

 arterioles and the fine radicals of the beginning veins is a network 

 of microscopical vessels, large enough in many places to admit of 

 but a single row of corpuscles and whose walls are composed of a 

 single layer of endothelial cells; these are the capillaries. 



THE HEART. 



The heart is a hollow, cone-shaped organ of muscle. It is situ- 

 ated in the cavity of the thorax, inclosed by a serous sac: the peri- 

 cardium. It lies between the lungs, rests on the diaphragm, and 

 is located more on the left than on the right side. It is placed 

 obliquely; its broad end, or base, by attachments to the blood- 

 vessels, is fixed to the front of the vertebral column. The base of 

 the heart extends from the fourth to the eighth dorsal vertebra. 

 The apex is inclined downward, forward, and to the left, where it 

 terminates just behind the interval between the fifth and sixth ribs, 

 % inch to the inner side of and 1% inches below the nipple. The 

 heart is 5 inches in length; in breadth, 3% inches; and in thick- 

 ness, 2~y 2 inches. 



The heart is brown in color, and on its surface has a longi- 

 tudinal and a transverse groove, which shows a division of the organ 

 in four parts: the two auricles and two ventricles. The heart 

 increases in all dimensions up to a late period in life, thus aug- 

 menting its weight. The auricles are cavities having thin walls. 

 The base of the heart is formed by the auricles. A partition 

 separates them and they are connected with the great veins, the 

 cavae and pulmonary veins, by which they receive blood coming 

 from every portion of the system. The aperture of communication 

 between the auricles and ventricles is the auriculo-ventricular open- 

 ing, which permits the blood to leave the auricle to enter the ven- 

 tricle, but valves prevent it from running back into the auricle. 

 The thick-walled parts of the heart are the ventricles, which become 

 thicker in the direction of the apex. Like the auricles, they are 

 separated by a partition and connected with the large arteries, the 

 pulmonary artery and aorta, by which they send blood to the 

 entire system. Both ventricles have valves called aortic and pul- 

 monary, which prevent the reflow of the blood from the arteries 

 into the ventricles. 



The right auricle consists of an oblong part, the sinus. The 



