24 MAN 



portant, as they form a capital standard of comparison 

 among the mammalia generally. The skull incloses and 

 protects the brain ; but it also bears the jaws, without which 

 it would be impossible to swallow the food preparatory to 

 digestion. Each of the jaws is fitted with teeth, which are 

 so arranged that the surfaces of those in the upper jaw 

 correspond with those in the lower. The top or crown of 

 the tooth is capped with enamel, the hardest substance in 

 the body. 



Teeth are of three kinds, viz., the incisors or cutting teeth, 

 canines or dog teeth, and premolars and molars or grinding 

 teeth. During life man has two sets of teeth ; the first, 

 called milk teeth, are twenty in number ; they fall out 

 during childhood, being replaced by the permanent teeth 

 to serve throughout the duration of life. The permanent 

 teeth are larger and stronger than the milk teeth and there 

 are thirty-two of them instead of twenty. An adult man 

 has two incisors, one canine, two premolars and three 

 molars on each side of the jaw, top and bottom. 



Respiration. Terrestrial creatures breathe air ; fishes, 

 too, breathe air, which is dissolved in the water. The 

 arterial blood pumped through the aorta is as pure in 

 quality as it is bright scarlet in colour. In its passage 

 through the body it parts with its life-giving qualities and 

 takes up the impurities given off by the worn-out tissues ; 

 and when it returns by way of the veins it is a dark purple 

 poisonous mixture, not only useless, but a positive danger 

 to the body. It is chiefly loaded with poisonous carbonic 

 acid gas, of which the venous blood must be cleansed 

 before it can again be put into circulation. The heart has 

 four chambers, each guarded by a valve. Pure blood passes 

 out of the left ventricle and venous blood returns into the 

 right auricle. By way of the pulmonary artery the right 

 ventricle sends the impure fluid to the lungs for purifi- 

 cation. 



By means of the nostrils and the mouth we take in 

 fresh pure air, of which about 22 per cent, is oxygen, 

 which is absolutely necessary to sustain life. At the back 

 of the mouth and the nasal passages is the windpipe, or 

 trachea, which passes into the thorax, or chest, where it 



