98 



NORMAL HISTOLOGY. 



FIG. 83. The neurilemma, or external in- 



vestment of the nerve-fibre, called 

 also the (t primitive sheath/ 7 or 

 " sheath of Schwann," is a thin, 

 homogeneous membrane enclosing 

 the medullary substance or myelin. 

 At regular intervals, upon the in- 

 ner surface of the neurilemma, and 

 surrounded by a small amount of 

 cytoplasm, are flattened, oval nu- 

 clei, which appear to belong to the 

 neurilemma. About midway be- 

 tween these nuclei the nerve-fibre 

 Meduiiated nerve-fibre. (Key j g CO nstricted, forming the " nodes " 



andRetzius.) A, node of Ran- 

 vier; B, nucleus belonging to of Ranvier. The neurilemma ap- 



*& 1U ft pears to pass through these nodes 

 distinct by the retraction of without interruption, so that the 



the myelin of the medullary ., . , . 



sheath, in the left-hand fig- neurilemma of one internode is 



ure the clefts of Lantermann con tinuoilS With that of the adja- 



are shown as white lines in the 



dark myelin. These figures are cent intemodes. At the nodes, 



ttSSfiS&SE and apparently within the neuri- 

 the fatty constituent of the lemma, is a disk, perforated for the 



myelin a dark brown or black. . 



passage ot the axis-cylinder, called 



the " constricting band " of Ranvier. It may be that this 

 band is of the nature of a cement-substance, joining the 

 neurilemma of neighboring internodes ; for the latter ap- 

 pear to be developed from cells, probably of mesoblastic 

 origin, which surround the nerve-fibres after their for- 

 mation, becoming flattened to form membranous invest- 

 ments of the nerve-fibre. If. this view be correct, the 

 neurilemma of each internode, with its single nucleus, is 

 to be regarded as a single, specialized cell, derived from 

 the surrounding connective tissues, and serving to protect 

 the nerve-fibre. In perfect harmony with this conception 

 of its nature are the facts that the nerves within the brain 

 and spinal cord are destitute of neurilemma, and that when 

 a nerve-fibre branches in its course the point of division 

 is always at one of the nodes of Ranvier (Fig. 83). 



The medullary sheath, or myelin, is a soft material inter- 



