AND CIRCULATION. 



89 



being thence distributed to the body, is returned again into 

 the dorsal vessel (Fig. 83), by fissures at its sides. 



236. In all the higher animals there is a central organ, 

 the heart, which forces the blood through the arteries to- 

 wards the surface, and receives it again on its return. 

 The HEART is a hollow muscular organ of a conical 

 form, which dilates and contracts at regular intervals, inde- 

 pendently of the will. It is either a single cavity, or is di- 

 vided by walls into two, three, or four compartments, as 

 seen in the following diagrams. These modifications are 

 important in their connection with the respiratory organs, and 

 indicate the higher or lower rank of an animal, as determined 

 by the quality of the blood distributed in those organs. 



237. In the mammals and birds the heart is divided by 

 a vertical partition into two cavities, each of which is 

 again divided into two compartments, one above the other 

 (Fig. 85). The two upper cavities are called auricles, and 

 the lower ones are called ventricles. Reptiles have two 



Fig. 85. 



Fig. 86. 



Fig. 87. 



auricles and one ventricle (Fig. 86). Fishes have one auri- 

 cle and one ventricle only (Fig. 87). 



238. The auricles do not communicate with each other, 

 nor do the ventricles. The former receive the blood 

 from the body and from the respiratory organs, and each 

 auricle sends it into the ventricle beneath, through an 

 opening guarded by a valve, to prevent its reflux ; while 

 the ventricles, by their contractions, force the blood through 

 the arteries into the lungs and through the body generally. 



8* 



