86 



The Vertebrate Organ Systems 



tures, the nails grow continuously due to the presence of a thickened 

 layer of germinativum cells at the base. In the typical hoof, the softer 

 subunguis fills in the spaces between the lateral edges of the unguis. 

 The pad thus formed is the "frog" which cushions the foot. Hoofs 

 are exceptionally well adapted for the support of these often heavy 

 animals. 



Teeth. — Although teeth are usually associated with the digestive 

 tract, they may be considered with the integument inasmuch as they 

 actually are derived from the stratum germinativum. Among the toothed 

 mammals, the teeth make their appearance at a very early stage when 

 cells from the germinativum layer push down into the dermis, forming 

 a ridge. Along this ridge, other groups of cells divide rapidly to 

 produce enamel organs. Gradually the teeth assume their typical form. 



DENTINE 



PULP CAVITY 



PULP CANAL 



Fig. 24. — Longitudinal section of a human molar tooth. 



Typically a mammalian tooth (Fig. 24) consists of the following 

 parts : the root imbedded in the jaw by various prongs, the crozvn 

 projecting above the jaw, and the neck or transitional region between 

 the crown and root. The teeth are fastened into the jaw by the cement 

 around the roots. Inside the tooth is the soft pulp cavity in which are 

 found the blood vessels and nerves. The hard portion of the tooth is 

 formed of dentine and enamel with dentine forming the bulk of the 

 structure. Dentine is nearly identical with bone in composition but 

 much denser. Tiny radiating canals permeate throughout the dentine. 

 Where the crown is exposed to constant use, the dentine is protected 

 by a layer of enamel, an exceptionally hard material. 



