THE TEETH 291 



inner angle of the tympanic bone, imme- 

 diately behind and to the inner side of the 

 foramen lacerum medium : through it the 

 Eustachian tube passes from the tympanic 

 cavity to the posterior narial chamber. 



vi. The external auditory aperture is the large 

 upwardly directed opening of the neck of the 

 tympanic bone, round the edge of which the 

 cartilage of the external ear is attached. 



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. 



The teeth. 



The teeth are cutaneous structures, developed 

 from the mucous membrane of the mouth. They 

 consist of ah 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 ia 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 



TT 2 



