24 



A COURSE ON ZOOLOGY. 



FIG. 12. 



Each half is separated horizontally into two compart- 

 ments, but these compart- 

 ments communicate by an 

 opening provided with a 

 valve. The upper cavities 

 are somewhat ear-shaped, 

 and they have been named 

 the right and left auricles, 

 while the lower chambers 

 are known as the right and 

 left ventricles. 



The walls of the auricles 

 are thin muscles that are 

 able to contract with great 

 force, and for this reason 

 the auricles appear very 

 small in a heart emptied of 

 blood. The ventricles, on 

 the contrary, have very 

 thick walls and are very 



SECTION OF THE HUMAN HEART (after 

 His). A, right auricle; B, right 

 ventricle ; C, left ventricle ; D, left 

 auricle; E, partition between the 

 two ventricles. Between the auri- 

 cles and ventricles, on right and fleshy; their tissue is a beau- 

 left, the tricuspid and mitral valves 

 with their cords and associated 



tiful red. The very thin but 

 muscles are shown. very strong valves which 



separate each auricle from 



the corresponding ventricle are held on their borders by 

 numerous tendinous fibres that are inserted in the mus- 

 cular walls of the ventricles. They can thus open in 

 only one direction, that which allows the blood to flow 

 from the auricles to the ventricles. These valves are ex- 

 ceedingly important organs ; on their regular and normal 

 action depends the accomplishment of a work in which 

 the least irregularity would imperil the existence of life. 

 The heart is the starting-point or the termination of 



