TISSUES OF SPECIAL FUNCTION. 



99 



posed between the neurilemma and axis-cylinder. It is not a 

 simple substance, but contains at least one constituent closely re- 

 sembling fat or oil in its chemical nature ; also a substance chemi- 

 cally allied to the keratin of horns and the superficial cells of the 

 epidermis, called neurokeratin ; and a homogeneous, clear fluid. 

 The way in which these constituents are combined is a matter of 

 doubt, the apparent structure of the medullary sheath varying 

 greatly when different modes of preparing the nerve for micro- 

 scopical study have been employed. But the neurokeratin appears 

 to exist as a delicate reticulum pervading the medullary substance. 

 The medullary sheath appears to be interrupted at irregular inter- 

 vals by oblique clefts, which surround the axis-cylinder like the 

 flaring portion of a funnel. These " Lantermann's " clefts are 

 occupied by a soft material, probably similar to that composing the 

 constricting bands (Figs. 84 and 85). 



FIG. 84. 



FIG. 85. 





v 



Ax.C. 



Fig. 84. Longitudinal view of portion of nerve-fibre from sciatic of dog. (Schiefferdecker.) 



S, neiirilemma ; T, stained substance'within the clefts of Lantermann. 

 Fig. 85. Cross-section from sciatic nerve of frog. (Bohm and Davidoff.) A, axis-cylinder, 



showing punctate sections of the fibrillse ; B, medullary sheath stained with osmic acid ; 



a, b, apparent duplication of the medullary sheath, due to the presence of a Lantermann 



cleft ; C, afeolar tissue between the fibres. 



The medullary sheath is developed after the formation of the 

 axis-cylinder, and is, at first, continuous along the course of the 

 latter. Subsequently it becomes interrupted at the nodes of 

 Ranvier by the constricting disk. It seems to be derived from 



