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SECTION YII 



THE NEEVOUS SYSTEM 



Revised for the Fifth Edition 

 By IRVING HARDESTY, A.B., Ph.D. 



PROFESSOR OF ANATOMY, THE TULAXE UNITERSITY OF LOUISIANA 



THE nervous system of man, both anatomically and functionally, is the most 

 highly developed and definitely distributed of all the systems of the body. 

 It consists of an aggregation of peculiarly differentiated tissue-elements, 

 so arranged that through them stimuli may be transmitted from and to all the 

 other tissue systems or functional apparatuses. It is a mechanism with parts so 

 adjusted that stimuli affecting one tissue may be conveyed, controlled, modified, 

 and distributed to other tissues so that the appropriate reactions result. While 

 protoplasm will react without nerves, while muscle will contract without the 

 mediation of nerves, yet the nervous system is of the most vital importance to 

 the higher organisms in that the stimuli required for the functioning of the organs 

 are so distributed throughout their component elements that the necessary 

 harmonious and coordinate activities are produced. For this purpose the 

 nervous system permeates every organ of the body; nerve cell-bodies, accu- 

 mulated into groups, receive impulses and give rise to the nerves which ramify 

 and divide into smaller and smaller branches till the division attains the individual 

 nerve-fibres of which the nerves are composed, and even the fibres bifurcate 

 repeatedly before their final termination upon their allotted elements. So 

 intimate and extensive is the distribution throughout that could all the other 

 tissues of the body be dissolved away, still there would be left in gossamer its 

 form and proportions — a phantom of the body composed entirely of nerves. 



The parent portion or axis of the system extends along the dorsal mid-line of 

 the body, surrounded by bone and, in addition, protected and supported by a 

 series of especially constructed membranes or meninges, the outermost of which 

 is the strongest. The cephahc end of the axis, the encephalon, is remarkably 

 enlarged in man, and is enclosed within the largest portion of the bony cavity, 

 the cranium, while the remainder of the central axis, the spinal cord, continues 

 through the foramen magnum and lies in the vertebral canal. 



The intimate connection of th,e axis with all the parts of the body is attained 

 by means of forty-six pairs of nerves, which are attached to the axis at somewhat 

 regular intervals along its extent. They course from their segments of attach- 

 ment through the meninges and through their respective foramina in the bony 

 cavity to the periphery. Of these craniospinal nerves, fifteen pairs pass 

 through the cranium and are attached to the encephalon, and thirty-one pairs 

 to the spinal cord. Some of the cranial nerves and all of the thirty-one pairs of 

 spinal nerves contain both afferent fibres, which convey impulses from the per- 

 ipheral tissues to the central axis, and efferent fibres, which convey impulses from 

 the axis to the peripheral tissues. The different pairs of nerves possess the two 

 types of fibres in varying proportions. 



Upon approaching the spinal cord, each spinal nerve is separated into two roots 

 ■ — its posterior or dorsal root and its anterior or ventral root. The afi^erent fibres 

 enter the axis by way of the dorsal roots, which are, therefore, the sensory roots, 

 and the efferent fibres leave the axis by way of the ventral or motor roots. 



As usually studied, the nervous system is referred to in two main divisions : — 



(1) The central nervous system, composed of — (a) The spinal cord, or medulla 

 spinalis, and (Jo) the brain or encephalon. 



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