178 



THE HUMAN BODY. 



B 



of the toes, are three to four feet long. If a fresh while nerve* 

 fibre be examined with the microscope it presents the appear- 

 ance of a perfectly homogeneous glassy thread ; but soon it 

 acquires a characteristic double contour (Fig. 

 78) from the coagulation of a portion of its 

 substance. By proper treatment with re- 

 agents three layers may be brought into view. 

 Outside is a fine transparent envelope (1, 

 Fig. 79) called the primitive sheath ; inside 

 this is a fatty substance, 2, forming the 

 medullary sheath (the coagulation of which 

 gives the fibre its double border), and in the 

 centre is a core, the axis cylinder, 3, which 

 is longitudinally fibrillated and is clearly the 

 essential part of the fibre, since near the end- 

 ing the primitive and medullary sheaths are 

 frequently absent. At intervals of about 

 one millimeter (fa inch) along the fibre are 

 found nuclei (c, Fig. 80), around each of 

 which lies a little protoplasm. These are 

 indications of the primitive cells which have 

 elongated and formed an envelope for the 

 axis cylinder, which itself is a branch given 

 off by a nerve-cell in some centre. The 

 medullary sheath is interrupted half-way 

 between each pair of nuclei at a point called 

 the node of Ranvivr (R, Fig. 80), which is 

 the boundary between two of the enveloping 

 cells. In the course of a nerve-trunk its 

 fibres rarely divide; when a branch is given 

 off some fibres merely separate from the 



f 



four hundred diame- ra fced out at one end into smaller bundles 



terse they nsivc ot*cn _^ _ . -. 



treated with, osmic containing fewer threads. .Near their ends, 



acid, which stains the , , -, , 



medullary sheath however, nerve-fibres frequently brancn, ana 

 8r^ **!& then a division of the axis cylinder goes to 



c, c, and nodes of , , , 



Ranvier, R. The axis each branch. 



Gray Nerve-Fibres. Some of these are 

 merely white fibres which near their peri- 

 pheral ends have lost their medullary sheaths; others have no 

 medullary sheath throughout their whole course, and consist 

 merely of an axis cylinder (often striated) and nuclei, with 



the nodes. 



