NERVOUS SYSTEM. 31 



A nerve trunk is composed of an indefinite number of nerve 

 fibers supported within a common shcatli. 



A nerve cell is an irregularly shaped microscopic cell, having 

 a varying number of branches, one of which, in case of the motor 

 cells, may continue to indefinite length as the axis cylinder or 

 central fiber previously mentioned. 



Nerve fibers are classified according to function into : motor, 

 sensory, and special sense. 



Motor fibers are those which convey impulses to the muscles 

 and control muscular action. 



Sensory fibers are those which convey impulses towarrl brain 

 centers and supply only sensation to the structures to which they 

 are distributed. 



Fibers of special sense, as in olfactory, optic, and auditory 

 nerves, transmit only sensations that pertain to the functions of 

 the special senses, like hearing and sight. 



Many of the cranial and facial nerves contain both motor 

 and sensory fibers and are therefore mixed. 



Nerve trunks are therefore classified as motor, sensory, mixed, 

 and nerves of special sense, according to the kind or kinds of 

 fibers composing them. 



The nervous system is subdivided for study into the cerebro- 

 spinal, and sympathetic nervous systems. 



CEREBRO-SPINAL SYSTEM. 



The cerebro-spinal nervous system consists of the brain and 

 spinal cord together, with their nerves and ganglia. The brain 

 and spinal cord should be considered as one complex organ lo- 

 cated within a continuous canal ; the brain to be considered as 

 merely an enlargement at the anterior extremity. The cranial 

 cavity should be considered as an enlargement at the anterior 

 extremity of the spinal canal. The cranial bones may be consid- 

 ered as peculiarly developed vertebrae. 



The brain is located in an ovoid cavity, the walls of which 

 are formed by the cranial bones. It is covered by three mem- 

 branes: (i) The dura mater, is tough, thick and strong, and 

 exactly fitted to the inner surface of the cranial bones. (2) The 

 arachnoid consists of two layers forming a closed sack, the outer 

 layer fitted closely to the dura mater, and the inner layer fitted 



