CHAPTER V 



THE SPINAL NERVES 



WE have had a glance at the earliest beginnings of a nervous system in the 

 animal series and learned something of its biologic significance. We have 

 traced briefly its development in the mammalian embryo, and become familiar 

 with its chief subdivisions. We have studied the microscopic units of which it 

 is composed, learning something of their development, structure, and function. 

 With this information we are prepared to take up a more detailed study of the 

 various subdivisions of the system. 



Subdivisions of the Nervous System. The most convenient and logical 

 classification of the parts of the nervous system is that which emphasizes the 

 distinction between the central organs and those peripheral portions which are 

 concerned chiefly in conducting impulses to and from the central organs, as 

 follows : 



The central nervous system: 

 Brain, 

 Spinal cord. 



The peripheral nervous system : 

 Cerebrospinal nerves: 

 Cranial nerves, 

 Spinal nerves. 



The sympathetic nervous system. 



The anatomic relationships of these subdivisions in man are illustrated in 

 Figs. 33 and 34. The brain lies within and nearly fills the cranial cavity. It is 

 continuous through the foramen magnum with the spinal cord, which occupies 

 but does not fill the vertebral canal. From the brain arises a series of nerves 

 usually enumerated as twelve pairs and known as cranial or cerebral nerves; 

 while thirty-one pairs of segmentally arranged spinal nerves take origin from the 

 spinal cord. 



Branches of the cerebrospinal nerves reach most parts of the body. They 



are composed of afferent fibers, which receive and carry to the central nervous 



system sensory impulses produced by external or internal stimuli, and of efferent 



fibers, which convey outgoing impulses to the organs of response. It is through 



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