406 PRACTICAL ZOOLOGY 



gnawing, thereby maintaining a serviceable length and a keen 

 cutting edge; insect-eating mammals, such as the shrew, seize 

 insects with their projecting incisors and cut them into pieces 

 with the pointed cusps on their premolars and molars; and man 

 and other omnivorous animals are provided with teeth fitted 

 for masticating both animal and vegetable matter. 



Circulation. The heart in mammals is more highly devel- 

 oped than in any other vertebrate. The ventricle is divided 

 into two chambers that are perfectly distinct. The pure blood 

 (in the pulmonary veins), passing from the lungs, enters the left 

 auricle, passes thence into the left ventricle, whence it is driven 

 (through the aorta) over the body. After having traversed 

 all the parts of the body and become richly loaded with carbonic 

 acid gas, it returns to the heart, entering the right auricle^ and 

 passing thence into the right ventricle, whence it is pumped 

 through the pulmonary arteries back into the lungs. Thus by 

 the division of the heart into two halves the arterial is com- 

 pletely separated from the venous blood. 



The blood corpuscles are unlike those of the lower vertebrates, 

 being smaller, round instead of oval, biconcave, and without 

 nuclei. The lymphatic system is of considerable importance 

 in mammals. The fluid portion of the blood, which, because 

 of the blood pressure, escapes through the walls of the capil- 

 laries into the spaces among the tissues, is collected into lymph 

 vessels. These vessels pass through so-called lymph glands 

 and finally empty into the large veins in the neck. The lym- 

 phatics which collect nutriment from the intestine are called 

 lacteals. 



Respiration. Mammals breathe air by means of lungs. 

 Tne trachea or windpipe is held open by incomplete rings of 

 cartilage ; and the larynx, or voice box, is supported by a num- 

 ber of cartilages, and across its cavity extend two elastic folds 

 called the vocal cords. 



The lungs are conical in shape, and lie freely in the thoracic 

 cavity. Air is drawn into them by the enlargement of the 



