THE NERVE FlBEIfc 



135 



latter is ruptured the myelin exudes 

 in the form of 'myelin drops.' The 

 myelin thus obtained possesses the 

 physical properties of a fat. It is 

 also capable of being blackened by 

 osmium tetroxid. By extraction with 

 ether the myelin can be removed, leav- 

 ing behind a supporting framework 

 of neurokeratin. The function of 

 myelin is probably nutritive, though 

 it has been regarded as an insulating 

 substance. It is thought to be present 

 in small amounts even in so-called 

 non-medullated fibers. It seems rea- 

 sonable to suppose that it may have 

 a double function, that is, nutritive 

 and in part insulating. 



At frequent intervals in the course 

 of the nerve fiber its myelin sheath 

 suffers complete interruption, thus 

 forming the annular constrictions or 

 nodes of Ranvier. At these points the 

 neurolemma dips in until it is in con- 

 tact with the axis cylinder. Both axis 

 cylinder and neurolemma are contin- 

 ued past the node without interrup- 

 tion. 



The successive nodes of Eanvier 

 divide the nerve fiber into internodal 

 segments. Within each internodal 

 segment the medullary sheath, on 

 blackening with osmium tetroxid, 

 presents clear intervals which pene- 

 trate the myelin sheath it such man- 

 ner as to give the appearance of 

 obliquely disposed clear lines or inci- 

 sions. These incisures of Schmidt 



FIG. 148. NERVE FIBERS. 



A and B, from the sciatic nerve of a 

 rabbit, isolated by teasing, and viewed 

 in profile; C, a group of nerve fibers in 

 transection, from the sciatic nerve of 

 a dog. a, axon; 6, neurolemma pro- 

 jecting beyond the torn end of the 

 fiber; d, nucleus; h, endoneurium or 

 fibrous sheath of Henle; Z, Schmidt- 

 Lantermann lines; n, nodes of Ran- 

 vier. Osmium tetroxid. A and B t 

 X 670; C, X 900. 



(Schmidt-Lantermann lines) have 



not been satisfactorily explained and can not be demonstrated in the 

 living fiber, yet they present a constant form and are always present in 

 10 



