io8 



ELEMENTS OF HISTOLOGY. [Chap. xiv. 



141. (6) The medullary sheath or white substance of 

 Schwann is also called the medulla of the nerve-fibre. 

 This is a glistening bright fatty substance surround- 

 ing the axis cylinder, as an insulating hollow cylinder 

 surrounds an electric wire. The medullary sheath 

 gives to the nerve-fibre its double or dark contour. 

 Between the axis cylinder and the medullary sheath 

 there is a small amount of albuminous fluid 

 which appears greatly increased 

 when the former, owing to shrink- 

 ing, stands farther apart from the 

 latter. 



142. (c) The sheath of Schwann, 

 or the neurilemma, closely surrounds 

 the medullary sheath, and forms 

 the outer boundary of the nerve- 

 fibre. It is a delicate structure- 

 less membrane. Here and there 

 between the neurilemma and the 

 medullary sheath, and situated in 

 a depression of the latter, is an 

 oblong nucleus, surrounded by a 

 thin zone of protoplasm. These 

 nucleated corpuscles are the nerve 

 corpuscles (Fig. 64A), and are analo- 

 gous 'to the muscle corpuscles, situ- 

 ated between the sarcolemma and 

 the striated muscular substance. 

 They are not nearly so numerous as 

 the muscle corpuscles. 



143. The neurilemma presents 

 at certain definite intervals annular 



constrictions, the nodes or constrictions of Ranvier (Figs. 

 64A,65,66), and at these nodes of Ranvier the medullary 

 sheath, but not the axis cylinder and its special sheath, 

 is suddenly interrupted and sharply terminates. The 

 portion of the nerve-fibre situated between two nodes is 



Fig. 64A. Two Nerve 

 Fibres, showing the 

 nodes or constric- 

 tions of Ranvier and 

 the axis cylinder. 

 The medullary 

 sheath has been dis- 

 solved away. The 

 deeply - stained ob- 

 lonir nuclei indicate 

 the nerve corpuscles 

 within the neuri- 

 lemma. (Atlas.) 



