VI HISTOLOGY OF THE FROG 131 



Each of the fields shows a dotted appearance, due to the 

 cut ends of the individual fibrillae. 



The muscle fibers of the heart differ from both of the 

 above classes. They are cross-striated, but each fiber con- 

 tains but a single nucleus. Each muscle cell is furnished 

 with branches which connect with the branches of contigu- 

 ous muscle cells, so that the whole mass forms a sort of 

 network. 



The tissue of the nervous system consists of nerve fibe7's, 

 and nerve, or ganglion cells. Each nerve is composed of 

 usually a large number of nerve fibers, held together by con- 

 nective nerve tissue and surrounded by a common sheath. 

 A typical nerve fiber presents the following parts : a central 

 strand, ox axis cylinder; a sheath of fatty substance around 

 this called the medullary sheath, or white substance of 

 Schwann ; and a delicate external membrane, neurilemma^ 

 or sheath of Schwann. At intervals constrictions occur, 

 called the nodes of Ranvier, where the white substance is 

 interrupted, although the axis cylinder and neurilemma are 

 continuous. Immediately beneath the neurilemma occur 

 the nuclei, each surrounded by a small amount of proto- 

 plasm. Each internodal segment, or space between two 

 nodes of Ranvier, contains several oblique markings across 

 the medullary sheath, which are known as the incisures of 

 Schmidt. 



The axis cylinder of a nerve is simply the elongated pro- 

 cess of a ganglion cell, and under high magnification is 

 found to be made up, much like a muscle cell, of very fine 

 fibrillae, with an intervening substance of more fluid con- 

 sistency. The white or medullary substance contains a 

 large amount of fatty material called myelin ; if a fresh nerve 

 is placed in water, this substance will swell up and collect 

 in drops, giving the nerve a very irregular outline. The 



