174 



THE HUMAN BODY. 



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. 71) from the- 

 coagulation of a portion of its substance. By proper treat- 

 ment with reagents three layers may be brought into view.. 

 Outside is a fine transparent envelope (1, Fig. 72) called 

 the primitive sheath j inside this is a fatty substance,, 2 r 



FIG 



FIG. 72. 



FIG. 71. White nerve-fibres soon after removal from the Body and when they 

 have acquired their double contour. 



FIG. 72. Diagram illustrating the structure of a white or medullated nerve- 

 $bre. 1, 1, primitive sheath ; 2, 2, medullary sheath ; 3, axis cylinder. 



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 clearly the essential 

 part of the fibre, since near its ending the primitive and 

 medullary sheaths are frequently absent. At intervals of 

 about one millimeter (^ inch) along the fibre are found 

 nuclei. These are indications of the primitive cells which 

 have elongated and formed an envelope for the axis cylin- 



