Chap. XIX] THE NERVOUS SYSTEM 367 



nerve-impulses in the contiguous process. This intermingling of 

 arborizations is called a synapse. 



(6) Axis cylinder process. — There is but one axis cylinder 

 process given off from each cell, and it differs from the dendrites 

 in the following particulars : — 



(1) It is usually longer; in some instances it travels as much 

 as 39.37 in. (1 meter) before breaking up into its terminal branches. 



(2) It has a smooth outline and diminishes in calibre very 

 little. 



(3) It gives off minute side branches called collaterals. These 

 are generally given off at right angles to the axis cylinder. 



(4) It merges into a nerve-fibre and usually becomes enveloped 

 in one or two coats. 



Function. — The function of the axis cylinder process differs 

 from that of the dendrites. The dendrites convey impulses to 

 the cell-body, and the axis cylinder process conveys impulses from 

 the cell-body. Some writers consider that the axis cylinder is 

 capable of carrying impulses in either direction, but this is not the 

 generally accepted view. 



(c) Nerve-fibre. — While the nerve-fibre is really only the con- 

 tinuation of the axis cylinder process that has undergone some 

 change in structure, it is advisable to describe the nerve-fibre 

 separately as though it were a new subject. 



Nerve-fibres are of two kinds : medullated, or white fibres, and 

 non-medullated, or gray fibres. 



Medullated fibre. — If one looks at a medullated nerve-fibre 

 under the microscope, it is found to consist of three parts : 

 (1) a central core called the axis cylinder which is a continuation of 

 the axis cylinder process ; (2) immediately surrounding the axis 

 cylinder is a sheath, or covering, of a semi-fluid, fatty substance 

 called the medullary, or myelin, sheath. It is to the refraction of 

 light from this fatty substance that medullated nerve-fibres owe 

 their white color ; (3) external to the medullary sheath is a thin 

 membrane completely enveloping the nerve-fibre and forming 

 the outer covering called the neurilemma. This is comparable 

 to the sarcolemma that invests muscle-fibres. 



Function of the medullary sheath. — It is supposed that the 

 medullary sheath serves: (1) as a source of nourishment, (2) as a 

 protection, and (3) as a non-conducting medium for the axis 



