30 



ZOOLOGY. 



extremity by an opening called the anus. Through 

 this tube the indigestible portions of the food are car- 

 ried off as excrement. 



27. Circulation. The heart, 

 which is surrounded by a mem- 

 branous sac called the peri- 

 cardium, contains four cham- 

 bers. The right side consists 

 of two cavities, the right auricle 

 and the right ventricle; the left 

 side has two similar cavities, 

 the left auricle and the left 

 ventricle. The blood passes 

 from the auricles to the ven- 

 Itricles; but, the apertures being 

 F guarded by valves, it cannot 

 1 pass from the ventricles to the 

 auricles. There is no com- 

 munication between the right 

 and left sides of the heart. 



Two great veins, the su- 

 perior and inferior venae caV8B, 

 receive the impure or venous 

 blood from the entire body, 

 and pour it into the right 

 auricle, from which it passes 

 to the right ventricle. From 

 this chamber it is driven to 

 the lungs through the pulmonary artery, where it is 

 changed by the oxygen of the air into arterial blood. 

 The pulmonary veins convey it back to the left auricle, 

 from which it passes to the left ventricle, and is by it 

 propelled through the aorta and its numerous branches to 

 the various parts of the body. 



The greater number of the corpuscles are red. 

 They are circular in form, except in the camels, and 

 are destitute of any solid particle or nucleus. The 

 blood is hot. 



Fio. V - -DIGESTIVE SYSTEM OF A 

 MAMMAL. 



e, rectum. 



