io8 



ELEMENTS OF HISTOLOGY. [Chap. xiv. 



141. (b) The medullary sheath or white substance 

 of Sch warm, also called the medulla of the nerve-fibre. 

 This is a glistening bright fatty substance surrounding 

 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, surrounds closely 

 the medullary sheath, and forms 

 the outer boundary of the nerve- 

 fibre. It is a hyaline delicate 

 membrane. From place to place 

 there is present between the neuri- 

 lemma and the medullary sheath, and 

 situated in a depression of the latter, 

 an oblong nucleus, surrounded by a 

 thin zone of protoplasm. These 

 nucleated corpuscles are the nerve 

 corpuscles (Fig. 6 4 A), and are analo- 

 gous to the muscle corpuscles, situated 

 between the sarcolemma and the 

 striated muscular substance. They 

 are not nearly so numerous as the 

 muscle corpuscles. 



143. The neurilemma produces 

 at certain definite intervals annular 



constrictions, the nodes or constrictions of Janvier (Figs. 

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

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

 suffers a discontinuity and sharply terminates. The 

 portion of the nerve-fibre situated between two nodes is 



v 



rig. 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- 

 long nuclei indicate 

 the nerve corpuscles 

 within the neuri- 

 lemma. (Atlas.) 



