NERVOUS TISSUE 



155 



The individual nerve fibers vary in diameter, and the larger ones are 

 probably those which have a longer course. It is impossible to distin- 

 guish histologically between sensory and motor fibers. The sheath of 



Endp- 



neurium. 



Medullary 

 sheath. 



Fiber sheath. 



FIG. 141. MEDULLATED NERVE. PART OF A CROSS SECTION OF THE HUMAN MEDIAN NERVE. X 220 



myelin which surrounds the fiber varies greatly in thickness, as seen 

 in the cross section, Fig. 141. In ordinary preparations it forms light 

 zones around the dark fibers, suggesting the relation between protoplasm 

 and nucleus; but the rod-like nature of the central fibers is evident on 

 changing the focus. The myelin is sur- 

 rounded by the membranous neurolemma, 

 within which the single internodal nucleus 

 is occasionally included in a given section. 

 Portions of isolated fibers, viewed longi- 

 tudinally, are shown in Fig. 142. 



N 



Myelin is a mixture of complex fats and lipoid 

 substances, some of which are combined with 

 sugar. Like fat, it is dissolved by ether and 

 blackens with osmic acid. In preserved speci- 

 mens the emulsion breaks down, giving rise to 

 various forms of shrinkage. A network which 

 appears after fibers have been treated with 

 alcohol and ether is said to be composed of 

 neurokeratin, a substance insoluble in these 

 reagents, which does not blacken with osmic 

 acid. The size of the meshes varies (Fig. 143, 

 A, B). In preparations blackened with osmic 

 acid, the myelin is often traversed by oblique 

 clefts, the incisures of Lantermann (Fig. 143, D). 

 The arrangement of these characteristic clefts 



may be pictured by imagining a succession of stemless funnels strung along the 

 axis cylinder, not all of which are pointed the same way. The incisures are doubt- 

 less artificial, and their number is increased by pulling the nerve fibers apart; they 

 appear to be empty or crossed by strands of myelin, but in the preparation shown in 



Nu. 



FIG. 142. Two NERVE FIBERS WITH 

 SHRUNKEN Axis CYLINDERS FROM 

 THE SCIATIC NERVE OF A RABBIT. 

 X 350. 



A, Axis cylinder; M, medullary sheath 

 (myelin); N, neurolemma; Nu, nu- 

 cleus of the neurolemma. 



