THE TEETH. 311 



vii. The foramen magnum is the large median 

 hole at the hinder end of the skull, surrounded 

 by the bones of the occipital segment. It is 

 traversed by the central nervous system, and 

 marks the boundary between the brain and 

 the spinal cord. 



6. 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 less hard 

 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. Sur- 

 rounding 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, 

 which is herbivorous, that the aperture at the base of 

 each does not become constricted, but remains wide 

 open, the teeth continuing to grow throughout life : 

 whereas in most mammals the base of the tooth 

 sooner or later becomes 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- 

 maxillee. 



