538 THE NERVOUS SYSTEM. 



pituitary glands. The latter has been supposed, from its 

 microscopic structure, to be rather a ductless gland (p. 410) 

 than a nervous organ. 



PHYSIOLOGY OF THE CEREBRAL AND SPINAL NERVES. 



/ 



The cerebral nerves are commonly enumerated as nine 

 pairs, but the number is in reality twelve, the seventh nerve 

 consisting, as it does, of two nerves, and the eighth of three. 

 These and the spinal nerves, of which there are thirty-one 

 pairs, symmetrically arranged on each side of what, re- 

 duced to its simplest form, may be regarded as a column 

 or axis of nervous matter, extending from the olfactory 

 bulbs on the ethmoid bone to the filwn terminale of the 

 spinal cord in the lumbar and sacral portions of the ver- 

 tebral canal. The spinal nerves all present certain cha- 

 racters in common, such as their double roots ; the isolation 

 of the fibres of sensation in the posterior roots, and those 

 of motion in the anterior roots ; the formation of the gan- 

 glia on the posterior root ; and the subsequent mingling 

 of the fibres in trunks and branches of mixed functions. 

 Similar characters probably belong essentially to the cere- 

 bral nerves; but even when one includes the nerves of 

 special sense, it is not possible to discern a conformity of 

 arrangement in any besides the fifth, or trifacial, which, 

 from its many analogies to the spinal nerves, Sir Charles 

 Bell designed as a spinal nerve of the head. 



According to their several functions, the cerebral or 

 cranial nerves may be thus arranged : 



Xerves of special sense . Olfactory, optic, auditory, part of the glosso- 

 pharyngeal, and the lingual branch of the 

 fifth. 



,, of common sensation. The greater portion of the fifth, and part of 

 the glosso-pharyngeal. 



