CHAPTEK XV 



PHYSIOLOGY OF NERVE 



MANY points relating to the physiology of iierve have been already 

 studied in connection with muscle. But there still remain further 

 questions upon which we have hardly touched as yet. 



Classification of Nerves. 



The nerve-fibres which form the conducting portions of the 

 nervous system may be classified into three main groups, according 

 to the direction in which they normally conduct nerve impulses. 

 These three classes are : 



1. Efferent nerve-fibres. 



2. Afferent nerve- fibres. 



3. Inter-central nerve-fibres. 



1. Efferent or centrifugal nerves are those which conduct im- 

 pulses from the central nervous system (brain and spinal cord) to 

 other parts of the body. When, for instance, there is a wish to move 

 the hand, the impulse starts in the brain, and travels a certain 

 distance down the spinal cord ; it leaves the spinal cord by one or 

 more of the spinal nerves, and so reaches the muscles of the hand 

 which are thrown into contraction. Such nerves are called motor, 

 but all efferent nerves are not motor ; some cause secretion instead 

 of movement, and others may cause a stoppage of movement, etc. A 

 list of the classes of efferent nerves is as follows : 



a. Motor. 



5. Accelerator. 



c. Inhibitory. 



d. Secretory. 



e. Electrical. 

 /. Trophic. 



a. Motor nerves. Some of these go to voluntary muscles ; others 

 to involuntary muscles, such as the vaso-motor nerves which 

 supply the muscular tissue in the walls of arteries. 



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