THE NERVE FIBRE 



115 



FlG. 112. A SMALL PORTION OF A TRANSECTION OF THE 

 SCIATIC NERVE OF A DOG. 



Nerve fibres are seen in transection ; their myelin 

 sheaths are black, their neuraxes unstained, 

 tetroxid. Photo, x VOO. 



Osmium 



The 



The axis cylinder presents a finely fibrillar structure. The 

 nature of these fibrils is not well understood. In certain nerve 

 fibres of the lower animals these fibrils have a tendency to collect 

 into the center of the 

 axis cylinder, leaving 

 a peripheral clear 

 zone ; this distribu- 

 tion is especially char- 

 acteristic of those 

 fibres which are not 

 supplied with a med- 

 ullary sheath. In 

 mammals, however, 

 the fibrillae occupy a 

 large portion of the 

 axis cylinder, the 

 clear peripheral area 

 being corresponding- 

 ly diminished until in man it can scarcely be recognized, 

 fibrils of the lower animals are also coarser. 



Apathy, studying chiefly the lower animals, has considered these 

 ? ultimate fibrtifo" to be the conducting element of the nerve 

 fibre. Others, however, lay greater stress upon the intervening 

 clear portion, the neuroplasm of Schiefferdecker, as containing 

 the active conducting element of the fibre. 



The axis cylinder is, under certain conditions, at least, found to 

 be inclosed by an extremely delicate membrane, the axilemma of 

 Kiihne. The existence of this membrane as an integral part of a 

 living axis cylinder has been denied by others. 



The medullary sheath (white substance of Schwann, myelin 

 sheath] forms a cylindrical investment for the axis cylinder. It 

 appears to be retained in position by the neurilemma, for when 

 the latter is ruptured the myelin exudes in the form of " myelin 

 drops." The myelin thus obtained possesses the physical proper- 

 ties of a fat. It is also capable of being blackened by osmium 

 tetroxid. By extraction with ether the myelin can be removed, 

 leaving behind a network of neurolceratin. It has not yet been 

 proved that this network exists in the living nerve fibre. 



At frequent intervals in the course of the nerve fibre its myelin 

 sheath suffers complete interruption, thus forming the annular 

 constrictions or nodes of Ranvier. At these points the neuri- 



