16 TEXT-BOOK OF ZOOLOGY 



the dentine in the form of folds, thereby imparting to the tooth surface 

 the necessary roughness (as in the molars of Eodents and Euminants). 

 As a rule these folds are filled by a bone-like material called cement. 

 This same substance in many cases also covers the fangs. The interior 

 of the tooth is always hollow, forming a cavity which encloses the nerves 

 and bloodvessels (for the nutriment of the tooth). The shape and 

 formation of the teeth bear the closest relation to the mode of life of 

 the animal, particular teeth, moreover, having special tasks assigned to 

 them. (Division of labour. Give examples.) 



5. Respiration is always carried on by means of lungs, and is per- 

 formed by the action of special' muscles. The most important of these 

 muscles is the diaphragm, which in the form of a partition curving 

 forwards (in man upwards) separates the thoracic from the abdominal 

 cavity. "When the diaphragm contracts, it becomes flatter, and so the 

 cavity is enlarged, and air is drawn into the lungs (inspiration). Expira- 

 tion is due chiefly to the contraction of the elastic pulmonary vesicles, 

 which are dilated by the inspired air. 



6. The Heart. — In mammals the heart shows its highest degree of 

 development. By a longitudinal section it is divisible into two halves, 

 each of which in turn consists of an anterior (auricle) and posterior 

 (ventricle) cardiac chamber. The pure blood (in the pulmonary veins, 

 L.V.), passing from the lungs (L.), enters the left auricle (I.V.), passes 

 thence into the left ventricle (I.K.), whence it is driven (through the aorta, 

 A.) 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 (by 

 the great veins, o.H. and u.H.), entering the right auricle (r.V.), and 

 thence passing into the right ventricle (r.K.), whence it is pumped 

 through the pulmonary arteries (L.A.) back into the lungs. Thus by the 

 division of the heart into two halves the arterial is completely separated from 

 the venous blood. 



7. The Urine. — The urine, excreted by the kidneys in mammals, is 

 conducted by two canals, the ureters, into the urinary bladder, which is 

 periodically emptied by the contraction of its walls. 



8. Reproduction.- — With the exception of the Cloacalia (which see), all 

 mammals bring forth living young, which, until they can feed themselves, 

 are sustained upon the milk of the mammary glands. The openings of 

 these glands generally lie upon wart-like elevations of the skin, the 

 nipples or teats. 



