THE HEART 



373 



between the blood tubes and the cells; these are filled 

 with lymph, a liquid much like the blood (cf. 379). 



375. The Heart. The heart (Fig. 284) lies in the cavi- 

 ty of the thorax, almost surrounded by the lungs. The 

 smaller end is to the left of the 

 breast bone, but the larger end 

 is directly behind it. The heart 

 of a person is about as large as 

 his fist. The heart proper is 

 surrounded by a protective cov- 

 ering with a double wall. This 

 is the pericardium, meaning 

 "around the heart." Between 

 the two halves of the pericar- 

 dium there is a liquid that de- 

 creases the friction of the heart 

 in its movements. 



The heart of man, like that 

 of all mammals (cf. 345), con- 

 sists of 4 cavities, or chambers. 

 These chambers are all needed, because the blood goes 

 through 2 distinct circuits, or "round trips. 77 One of 

 these circuits is from the heart to the body, and then 

 back to the heart. This is the system, or body, circula- 

 tion. The other circuit is from the heart to the lungs, and 

 back again to the heart. This is the lung, or pulmonary, 

 circulation. The two upper cavities of the heart are the 

 right auricle and the left auricle (compare again the heart 

 of the crayfish, fish, frog, and bird). The auricles receive 

 the blood from the body and lungs. The heart's two 



Right Ventricle 



FIG. 284. 



The Heart. Veins bring blood to 

 the auricles, and arteries carry it 

 away from the ventricles. Note 

 the valves, and the means of hold- 

 ing them shut. 



