n] 



HISTOLOGY 



23 



veniently described in dealing with the nervous system. Here 

 it will suffice to give a brief account of the different kinds of 

 nerve fibres and the cells from which they arise. 



If we cut across a nerve trunk, and examine a section of it 

 under the microscope we find that it is made up of a large number 

 of nerve fibres M'hich are held together by connective tissue. If 

 we confine our examination to one of these fibres we find that it 

 consists of a central strand or core, known as the axis cylinder 

 or axon, and an outer portion consisting of the medullary sheath 

 and the neurolemma, the latter being a fine membrane which 

 surrounds the sheath. The axon is concerned with the conduc- 



"-^Siwair.: 



Fig. 18. Non-medullated nerve fibres from Nissl preparation of vagus of cat 

 (photograph from Schafer). 



tion of the nerve impulse ; the medulla, which is not quite com- 

 plete but is broken at intervals, is of the nature of a protective 

 covering and is composed of a phosphorised fatty substance. 

 Such nerve fibres are called medullated fibres . Intermingled with 

 these are other fibres in which the medullar^' sheath is lacking. 

 These are called non-medullated fibres. They are especially 

 common in the so-called sympathetic nerves. These non- 

 medullated fibres appear to possess numerous nuclei, but the 

 nuclei almost certainly belong actually to a very thin investing 

 sheath. 



