i o 2 Life and He a Ilk 



a cavity. This is the pulp cavity, which is filled with the 

 dental pulp, consisting chiefly of nerves and blood vessels. 

 Irritation of this delicate pulp, due to the decay of the 

 dentine, gives rise to toothache (Fig. 48). 



Surrounding the cavity on all sides is the hard substance 

 known as the dentine, or ivory. Outside the dentine of 

 the root is a substance closely resembling bone, called 

 cement. The root is held in its socket by fibrous membrane 

 which surrounds the cement as the periosteum does bone. 



The crown of the tooth is not covered by cement, but 

 by the hard enamel, which forms a strong protection for 

 the exposed part. 



FIG. 47. Diagram of the Structure of 

 Secreting Glands. 



A, simple tubular gland ; B, gland with mouth shut and sac formed ; C, gland 



with a coiled tube ; >, plan of part of a racemose gland. 

 f 



^ 154. The Salivary Glands. While the food is being 

 chewed it is moistened with a fluid called saliva, which flows 

 from six little glands, three on each side of the mouth. The 

 parotid is situated in front of the ear. In the disease known 

 as "mumps," common in childhood, this gland becomes 

 inflamed and swollen. The submaxillary gland is placed on 

 the inner side of the lower jaw ; the sublingual is on the floor 

 of the mouth, between the tongue and the gums. Each 

 gland opens into the^nouth by a little duct (Fig. 49). 

 1 155. The Saliva. The saliva is a colorless liquid with- 

 out taste or smell. Its principal constituent, besides water, 

 is a ferment called ptyalin, which has the remarkable 



