CH. VII.] 



NERVE-FIBRES 



91 



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 com- 

 munication 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 w^ say, the hand has lost sensibility. 



This distinction between efferent and afferent 

 nerves is a physiological one, which we shall work 

 out more thoroughly later on. No histological dis- 

 tinction 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-Jibres. 

 These become sheathed in a 

 manner to be immediately de- 

 scribed, and are contained in the 

 nerves, and in the white matter of 

 brain and spinal cord. The bodies 

 of nerve-cells differ in size, shape, 

 B and arrangement, and we shall 

 discuss these fully when we get 

 to the nerve-centres. For the 

 present it will be convenient to confine ourselves to 

 the nerve-fibres as they are found in a nerve. 



Nerve-fibres are of two histological kinds, medul- 

 lated and non-medullated. Medullated nerve-fibres 

 are found in the white matter of the nerve-centres 

 FIG us Nerve- an( ^ ^ n ^ ne nerves originating from the brain and 

 fibre stained with spinal cord. Non-medullated nerve-fibres occur in 



osmic acid. A, f ., . . 



node ; B, nucleus, the sympathetic nerves. 



(Key and Tlie medullated or w hite fibres are characterised 



FIG. 119. A node of Ranvier 

 in a medullated nerve-fibre, 

 viewed from above. The 

 medullary sheath is in- 

 terrupted, and the primi- 

 tive sheath thickened. 

 Copied from Axel Key and 

 Retzius. x 750. (Klein 

 and Noble Smith.) 



