424 THE NERVOUS SYSTEM. 



To the fornix and other commissures no special function can 

 be assigned ; but it is a reasonable hypothesis that they con- 

 nect the action of the parts between which they are severally 

 placed. 



As little is known of the function of the pineal and pitu- 

 itary glands. The latter has been supposed, from its micro- 

 scopic structure, to be rather a ductless gland (p. 325) 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 consist- 

 ing, 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, reduced 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 filum terminate of the spinal cord in the lumbar 

 and sacral portions of the vertebral canal. The spinal nerves 

 all present certain characters 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 ganglia on the posterior root ; and the subsequent min- 

 gling of the fibres in trunks and branches of mixed functions. 

 Similar characters probably belong essentially to the cerebral 

 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 anal- 

 ogies to the spinal nerves, Sir Charles Bell designated as a 

 spinal nerve of the head. 



According to their several functions, the cerebral or cranial 

 nerves may be thus arranged: 



Nerves of special sense, . . Olfactory, optic, auditory, part of the 



glosso-pharyngeal, and the lingual 

 branch of the fifth. 



Nerves of common sensation, Tho greater portion of the fifth, and 



part of the glosso-pharyngeal. 



Nerves of motion, .... Third, fourth, lesser division of the 



fifth, sixth, facial, and hypoglossal. 



Mixed nerves, Pneumogastric and accessory. 



The physiology of the several nerves of the special senses 

 will be considered with the organs of those senses. 



