DIGESTION. 



number, 3 on each side above and below. Their large crown and 

 their great width are the chief distinguishing characteristics. The 

 upper molars have 3 conical fangs, the lower ones 2. The last molar 

 is the wisdom tooth, so called because it appears about the twentieth 

 year, when the individual is assumed to have acquired wisdom. The 

 molars are intended for the grinding of food. 



Structure of the Teeth. If a tooth is split in its long axis the 

 surface exhibits, besides the pulp-cavity, three different kinds of 

 materials. Dentine forms the greater part of the yellowish-white 



S. D 



A , 



., . , --- r^^h 



Fig. 8. Portion of the Crown of a Longitudinal Section of a Human 

 Premolar. X 200. (SoeoTTA.) 



The figure shows the structure of a tooth at the border of enamel and dentine. 

 In the region of the dentine two larger and two smaller interglobular spaces 

 are shown. The dentinal fibers branch and fork and with their processes pass 

 beyond the limits of the enamel. In the figure, the enamel prisms show partly 

 wavy curves and partly alternating stripes of darker and brighter prisms (the 

 parallel stripes of Retzius). D, Dentine. Dk, Dentinal tubules. Jg, Inter- 

 globular spaces. 8, Enamel. Sp, Enamel prisms. 



substance ; the capping of the crown is enamel ; and the translucent, 

 thin investment on the fang is cement, or crusta petrosa. 



The main bulk of the tooth is composed of dentine, giving it 

 shape and containing the pulp-cavity. It consists of about 28 parts 

 of organic matter and 72 of earthy material. Dentine resembles bone 

 both physically and in chemical constitution. When subjected to 

 microscopical examination we find the dentine penetrated throughout 



