CHAPTEE VII 



NEBVE 



NERVOUS tissue is the material of which the nervous system is com- 

 posed. The nervous system is composed of two parts, the central 

 nervous system, and the peripheral nervous system. The central nervous 

 system consists of the brain and spinal cord ; the peripheral nervous 

 system consists of the nerves, which conduct the impulses to and from 

 the central nervous system, and thus bring the nerve-centres into 

 relationship with other parts of the body. 



Some of the nerves conduct impulses from the nerve-centres and 

 are called efferent ; those which conduct impulses in the opposite 

 direction are called afferent. When one wishes to move the hand, the 

 nervous impulse starts in the brain and passes down the efferent or 

 motor nerve-tracts to the muscles of the hand, which contract; when 

 one feels pain in the hand, afferent or sensory nerve-tracts convey an 

 impulse to the brain which is there interpreted as a sensation. If all 

 the nerves going to the hand are cut through, all communication 

 with the nerve-centres is destroyed, and the hand loses the power 

 of moving under the influence of the will, and the brain receives 

 no impulses from the hand, or, as we say, the hand has lost 

 sensibility. 



This distinction between efferent and afferent nerves is a physio- 

 logical one, which we shall work out more thoroughly later on. No 

 histological distinction can be made out between motor and sensory 

 nerves, and it is histological structure which we wish to dwell upon 

 in this chapter. 



Under the microscope nervous tissue is found to consist essen- 

 tially of nerve-cells and their branches. The nerve-cells are contained 

 in the brain and spinal cord, and in smaller collections of cells on 

 the course of the nerves called ganglia. The part of the nerve- 

 centres containing cells is called grey matter. 



Long branches of the nerve- cells are known as nerve-fibres. 

 These become sheathed in a manner to be immediately described, 

 and are contained in the nerves, and in the white matter of brain and 



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