136 



NERVOUS TISSUES 



osmic preparations. These iiicisures subdivide the interannular segments 

 of the medullary sheath into medullary segments. Schmidt originally 

 considered them to be the optical expression of folds in the outer fibrous 

 coats. Lantermann and others claim to have shown that they are within 

 the neurolemma. They are believed by others to represent the limits of 

 cones of neurokeratin. The incisures may point in different directions. 

 They are more probably artifacts, representing fractures in the delicate 

 myelin sheaths. 



In preparations of fresh nerve fibers which have been treated with 

 silver nitrate according to the method of Banvier, the solution is found 



to enter the fiber most readily 

 at the nodes of Ranvier, so that 

 if blackened by exposure to the 

 sunlight, minute -{--like appear- 

 ances are seen at each node. By 

 prolonged maceration in weak 

 solutions of silver nitrate the so- 

 V lution penetrates still farther 



FIG. 149. CROSS AND LONGITUDINAL SEC- 

 TIONS OP THE SAME FUNICULUS (N) 

 OF NON-MEDULLATED NERVE FlBERS 

 (TURNED UP AT THE LEFT), SHOWING 

 THE PERINEURIUM (P) AND THE RELA- 

 TIONSHIP OF THE NEUROLEMMA NUCLEI 

 TO THE Axis CYLINDER BUNDLES OF 

 NEUROFIBRILS. 



From the peritracheal areolar tissue of 

 the cat. X 1500. 



and the "blackened axis cylinder 

 is found to possess spiral trans- 

 verse markings which are quite 

 characteristic. The true mean- 

 ing of these appearances has not 

 been satisfactorily explained. 

 Because of the apparent greater 

 permeability of the fiber at these 

 points, these peculiarities have 



been taken to indicate a certain relation of the annular constrictions to 

 the nutrition of the fiber. 



THE NEUROLEMMA (Nucleated Sheath of Scliwann). The neuro- 

 lemma is the outermost of the nerve fiber sheaths. It is of ectodermal 

 origin and makes its appearance prior to the medullary sheath. It 

 forms a very delicate membrane, which incloses the myelin substance, 

 and at each node of Ranvier com^s into contact with the axis cylinder. 

 Attached to the inner surface of the neurolemma in each internode, 

 and usually but one for each internodal segment, is an oval nucleus. 

 The nucleus is surrounded by a minute amount of finely granular cyto- 

 plasm. This structure is taken to indicate that the embryonal neuro- 

 lemma is formed by cells which became spread out over the surface of 

 the primitive fiber, one cell, as a rule, supplying each internodal segment ; 



