132 



THE BIOLOGY OF THE FROG 



CHAP, 



medullary sheath is supposed to act as a sort of insulator, 

 like the coatings that are wound around an electric wire. 



The nerve fiber, unhke that of muscle, is a composite 

 structure, being formed of cellular elements of diverse 

 origin. The sheaths of the nerve represent a series of cells 

 which have become applied to but have an entirely different 

 origin from the axis cylinder. The latter is always an out- 

 growth of a nerve or ganglion cell and is always of ectoder- 

 mic origin. In the development of a nerve the axis cylinder 

 is always the first part to make its appearance ; as it grows 

 out, pushing its way through the other tissues, it becomes 



ABC 



-IS. 



^x 



y 



m. 



Fig. 37. — Nerve cells and fibers of / the frog. A, fresh nerve fiber. B, 

 nerve fiber with the myelin swollen through the absorption of water. 

 C, cross section of nerve fibers. D, ganglion cells, ax, axis cylinder; 

 ax.p, axis cylinder process of ganglion cell; /.S, incisure of Schmidt; 

 m.s, medullary sheath; n, nucleus; fi/, neurilemma; N.J^, node of Ran- 

 vier; p./>, protoplasmic process of ganglion cell. 



surrounded with nucleated cells which flatten out and form 

 the neurilemma; the white substance appears at a compara- 



