XERVE TISSUE 



139 



usually proportionate to the size of the fibre, the 

 smaller fibres having the shorter internodes. In 

 fresh specimens the medullary sheath of an inter- 

 node appears continuous (Fig. Si, A), but in fixed 

 specimens it is broken up into irregular segments, 

 Schmidt-Lantermann segments, by clefts which 

 pass from neurilemma to the axolemma or axone, 

 and are known as the clefts or incisures of Schmidt- 

 Lantermann (Fig. 8i, C). On boiling medullated 

 nerve fibres in alcohol and ether a fine network 

 is brought out in the medullary sheath, the 

 neurokeratin network. Owing to the resistance of 

 neurokeratin to the action of trypsin, it has been 

 considered as possibly similar in composition to 

 horn. 



(3) The neurilemma or sheatJi of Schwann 

 (Figs. 86, B, and 85) is a delicate structureless 

 membrane which encloses the myelin. At the 

 nodes of Ranvier the neurilemma dips into the 

 constriction and comes in contact with the axone 

 or axolemma. Silver nitrate staining shows a 

 black transverse line in the neurilemma at the 

 node of Ranvier. This line represents the bound- 

 ary between two neurilemma cells. Against the 

 inner surface of the neurilemma, usually about 

 midway between two nodes, is an oval-shaped 

 nucleus, the nucleus of the neurilemma (Figs. 80, 

 C, and 85). Each nucleus is surrounded by an 

 area of granular protoplasm, and makes a little 

 depression in the myelin and a slight bulging of 

 the neurilemma (Fig. 81, C). According to most 

 observers no neurilemma is present in the central 

 nervous system. An important exception is 

 Cajal, who describes the medullated fibre of the 

 central nervous system as having a neurilemma. 



In addition to the above-described sheaths, 

 most medullated fibres of peripheral nerves have, 



Fig. 83. — Dia- 

 grammatic Repre- 

 sentation of Portions 

 of Two Medullated 

 Nerve Fibres, as seen 

 in Longitudinal Sec- 

 tion, stained with 

 OsmicAcid. (Length 

 of internode is pro- 

 portionately short- 

 ened.) i?,7?, Nodes 

 of Ranvier, with axis 



cylinder passing through; a, neurilemma; c, nucleus surrounded by protoplasm, 

 King at about the middle of the internode between the neurilemma and the medullary 

 sheath. (From drawing by J. E. Neale in Quain's Anatomy.) 



