TISSUES OF SPECIAL FUNCTION. 97 



ure running a continuous course from the cell giving it origin to 

 the peripheral termination of the nerve, called the "axis-cylin- 

 der " ; an external membranous envelope, the " neurilemma " ; and 

 a semisolid material, the "myelin," "white substance of Schwann," 

 or " medullary sheath," lying within the neurilemma and surround- 

 ing the axis-cylinder. 



The axis-cylinder is a greatly elongated process (axis-cylinder 

 process) springing from a nerve-cell. It is marked by longitudinal 



FIG. 82. 



Small ganglion in the tongue of a rabbit: a, a', ganglion- cells ; a', cell, with the beginning 

 of its axis-cylinder process ; 6, medullated nerve-fibre in cross-section ; c, fibrous tissue 

 within the ganglion (part of this fibrous structure may be composed of non-medullated 

 nerve-fibres) ; d, areolar tissue surrounding the ganglion and containing adipose tissue in 

 the upper and lower parts of the figure. To the left is a striated muscle-fibre. The gan- 

 glion is seen in cross-section, so that its connection with the nerves, in the course of which 

 it lies, is not visible. 



striations, which appear to represent exceedingly delicate fibrils 

 composing the axis-cylinder. These fibrils frequently separate at 

 the distal extremity of the nerve and take part in the construction 

 of the various forms of nerve-endings. A more minute study of 

 the axis-cylinder leads to the inference that it is composed of 

 spongioplasm, continuous with that of the body of the cell, and 

 that the appearance of longitudinal striation is due to the elongated 

 shape of the spongioplasmic mesh work and the greater thickness 

 of its longitudinal threads, the transverse threads uniting them 

 being much less conspicuous. 



