CHAPTER VIII 

 THE BRAIN 



The student should now be able to picture to himself 

 the following large features of the central nervous sys- 

 tem of man. The brain is enclosed within the skull and 

 the spinal cord descends from it through a bony canal 

 formed by the arches of the vertebrae. From the cord 

 spring to right and left 31 pairs of nerves. Each one 

 is a cable composed of thousands of fibers. Somewhat 

 more than half these fibers are arranged to convey 

 impulses from receptors to the cord; the remainder 

 are used to carry impulses out to muscles and glands. 

 (Muscles and glands are sometimes called effectors.) 



Close to the cord each spinal nerve cleaves into a 

 dorsal and a ventral root. The dorsal root bears a 

 ganglion containing the cells with which the afferent 

 fibers are united. The ventral root is made up almost 

 wholly of efferent fibers. It will be seen that a spinal 

 nerve, considered external to the place of union of its 

 two roots, is mixed in character, containing both af- 

 ferent and efferent fibers. When a nerve branches, 

 its fibers do not branch but are merely parted. Yet 

 nerve fibers may branch, as we have seen, within the 

 central nervous system and also, in the case of the 

 motor elements, when they approach the end-plates 

 in skeletal muscle. 



As there are spinal nerves springing from the cord so 

 there are nerves which originate from the brain. These 

 are known as the cranial nerves. They emerge through 

 openings in the skull and are distributed to localities in 

 the head and neck. One of them, on each side, departs 

 from this rule since it runs down into the trunk. This is 



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