TEXT-BOOK OF ZOOLOGY 



(c) Sensations of touch are conveyed by the whole skin ; as special 

 organs of touch we may enumerate the tips of the fingers (in man, 

 apes) ; the lips, with their special bristles (examples ?) ; the trunk 

 (examples ?) ; the wing-membrane in bats ; as well as the tongue 

 (giraffe, cattle). 



(d) The nose is the organ of smell. Its interior consists of large 

 cavities lined by a membrane. Very fine rod-like terminations of the 

 nerve of smell (olfactory nerve) project through the nasal membrane 



into the interior of the cavities, 

 and are fanned and stimulated 

 by the current of air which 

 enters the nose during inspira- 

 tion. This stimulus is trans- 

 mitted by the nerve to the 

 brain. To prevent the nerve 

 terminations from drying up and 

 becoming functionless, they are 

 kept constantly moist by a 

 mucous fluid secreted by the 

 nasal mucous membrane. (It 

 is impossible to smell when the 

 nose is dry.) 



(e) The tongue is provided 

 with organs of taste. Micro- 

 scopically fine terminations of 

 the nerves of taste (gustatory 

 nerves) are chemically stimulated 

 by sapid substances. These 

 nerve terminations also require 



SECTION THROUGH HUMAN INCISOR (LOWER 

 JAW). 



K, Lower jaw (mandible) ; H., cavities in the lower to be kept constantly moist 

 jaw, with parts of penetrating arteries ; Zf. , gum ; , . N 



L., lower lip ; L.M., muscular fibres of the lip ; (saliva). 



Z.b., dentine; Z.scli., enamel; C., cement; 4. Teeth. Most mammals 



N., nerve; B.. bloodvessels which enter the . n , . , . . , ., 



dental cavity. are provided with teeth in both 



jaws (see p. 11). These are 



fixed tightly in special pits in the jaws, the teeth sockets or alveoli. 

 The portion of the tooth which projects freely from the jaws is termed 

 the crown. If the part of the tooth which is embedded in the jaw is 

 clearly distinguishable from the crown it is spoken of as the fang ; if 

 otherwise, the tooth is said to "befangless. 



The teeth are composed of a hard bone-like substance, the dentine or 

 ivory. The crown is generally covered by a material of glassy hardness, 

 the enamel (e.g., in man). Frequently, too, the enamel penetrates into 



