ANATOMY OF THE NERVOUS SYSTEM 139 



system has numerous functions in addition to the simple one of 

 making connections between sensory and motor neurons. These 

 special functions have to do with the modification of the impulses 

 passing through it for the best advantage of the organism as a 

 whole.) 



Springing from the central nervous system and corresponding 

 to the cables bearing wires to individual telephones are forty- 

 three pairs of nerve-trunks. Twelve pairs arise from the brain and 

 are called cranial nerves; the remaining thirty-one pairs arise from 

 the spinal cord and are called spinal nerves. Each nerve-trunk 

 contains a large number of axons, and in most nerve-trunks the 

 axons of both motor and sensory neurons are present. These 

 forty-three pairs of nerve-trunks with their ramifications to all parts 

 of the Body constitute the peripheral nervous system (Fig. 55). 



There are in the Body a set of neurons which though part of 

 the peripheral nervous system are specially adapted for a certain 

 function and are therefore usually considered independently. 

 These constitute the sympathetic or autonomic system. 



The Central Nervous System and its Membranes. Lying in- 

 side the skull is the brain and in the neural canal of the verte- 

 bral column the spinal cord, the two being continuous through 

 the foramen magnum of the occipital bone. The central nervous 

 system is bilaterally symmetrical throughout except for slight 

 differences on the surfaces of parts of the brain, which are often 

 found in the higher races of mankind. Both brain and spinal cord 

 are very soft and easily crushed; nervous tissue as well as the con- 

 nective tissue and a peculiar supporting tissue (neuroglia) which 

 pervades it being delicate; accordingly both organs are placed in 

 nearly completely closed bony cavities and are also enveloped by 

 membranes which give them support. These membranes are 

 three in number. Externally is the dura mater, very tough and 

 strong and composed of white fibrous and elastic connective tis- 

 sues. In the cranium the dura mater adheres by its outer surface 

 to the inside of the skull chamber, serving as the periosteum of its 

 bones; this is not the case in the vertebral column, where the 

 dura mater forms a loose sheath around the spinal cord and is 

 only attached here and there to the surrounding bones, which 

 have a separate periosteum of their own. The innermost mem- 

 brane lies in immediate contact with the proper nervous parts. 



