THE TEETH 283 



traversed by the central nervous system, and 

 marks the boundary between the brain and 

 the spinal cord. 



The teeth. 



The teeth are cutaneous structures, developed 

 from the mucous membrane of the mouth. They 

 consist of an extremely hard outer layer of enamel, 

 which is epidermal in origin ; and a central portion 

 of dentine, which is developed from the dermis. The 

 dentine is hollow, its cavity containing the pulp of 

 the tooth, which receives vessels and nerves through 

 the large aperture at its base. Surrounding the 

 lower part of the tooth, and filling up the folds of 

 its surface, is a thin layer of bone, the crusta petrosa 

 or cement. 



It is characteristic of the teeth of the rabbit that 

 the aperture at the base of each does not become 

 constricted, but remains wide open, the teeth con- 

 tinuing to grow throughout life ; whereas in most 

 mammals the base of the tooth sooner or later be- 

 comes narrowed to form a root or fang, the aperture 

 becoming almost obliterated, and the growth of the 

 tooth ceasing entirely. 



The teeth of the rabbit do not form a continuous 

 series as in man, but the front teeth or incisors are 

 separated by a great gap or diastema from the group 

 of grinding teeth in the hinder part of the mouth. 

 The rabbit has no teeth corresponding to the canine 

 teeth of other mammals. 



a The teeth of the upper jaw. 



i. The incisors are two pairs of teeth at the front 

 of the upper jaw, lodged in alveoli in the pre- 

 maxillae. 



a. The anterior incisors are long, and curved 

 almost into semicircles, the greater part of 

 each being lodged in an alveolus which 

 extends almost to the hinder end of the 



