NERVOUS SYSTEM 



35 



are distributed ; for example, irritation of a sensory nerve gives 

 an impression of pain, but no motion or special sensation. 



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

 nerves, transmit only sensations that pertain to the functions of 

 the special senses like hearing 

 and sight. A jar on the head 

 irritating the optic nerve gives 

 a sensation of flashes of light. 

 Irritation of the auditory nerve 

 gives an impression of sound, etc. 



Many of the cranial and all the 

 spinal nerves contain both motor 

 and sensory fibers and are there- 

 fore mixed. 



Nerves are therefore classified 

 as motor, sensory, and mixed 

 nerves, 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. But these are very 

 intimately related to each other. Working together they con- 

 stitute one united system. 



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Fig. 20. 



Cerebrospinal System 



The cerebrospinal 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 

 located 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 considered as peculiarly developed vertebra?. 



Brain. — This organ is located in an ovoid cavity, the walls of 

 which are formed by the cranial bones. It is covered by three 

 membranes : (1) The dura mater is tough, thick, and strong, and 

 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 closely to the pia mater. (3) The pia mater is thin, 



