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AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



tions, such as those of the alimentary canal, circulatory organs, 

 &c., are mainly effected. It is, however, quite subordinate to 

 the brain and spinal cord. 



Sense Organs. (1) Tactile Organs. The sense of touch is pos- 

 sessed by the skin generally, and the mucous membrane of the 

 olfactory sacs and mouth-cavities. The nerve-fibres connected 

 with this sense are derived from the trigeminal in the head and 

 spinal nerves in the rest of the body. They break up into 

 plexuses, and in the dermis numerous groups of oval flattened 

 touch-corpuscles are present. These are most numerous beneath the 

 wart-like elevations, and nerve-fibres are continuous with them. 



(2) Gustatory Organs. Scattered throughout the mouth-cavity, 

 and especially numerous around the vomerine teeth, and on the 

 fungiform papillae of the tongue, are small groups of elongated 

 forked gustatory cells. They belong to the epithelium, and fibres 

 of the glossopharyngeal nerves terminate in them. 



(3) Olfactory Organs. Two olfactory sacs (Fig. 51), separated 

 by a median partition, the nasal septum, are lodged in the front 

 of the head. Each contains a complicated internal cavity, com- 

 municating with the exterior and mouth-cavity by the external and 

 internal naris respectively, and with glandular walls largely lined 

 by olfactory epithelium, containing numerous spindle-shaped olfac- 

 tory cells (Fig. 62), in which fibres of the olfactory nerve end. 

 From these cells a stiff process or bundle of olfactory hairs projects 

 into the nasal cavity. 



(4) Auditory Organs. The ear on each side is made up of 

 two parts (1) Middle ear, and (2) Internal ear. 



