296 THE HUMAN BODY. 



which is in proportion very great. Each is continuous 

 from a nerve-centre to the region in which it ends, so 

 that the fibres, e.g., which pass out from the spinal cord 

 and run on to the skin of the toes, are three to four 

 feet long. If a perfectly fresh white nerve-fibre be exam- 

 ined with the microscope it presents the appearance of a 

 homogeneous glassy thread ; but soon it acquires a charac- 

 teristic double border (Fig. 85) from the coagulation of a 

 portion of its substance, as a result of which three layers 

 are brought into view. Outside is a thin transparent en- 

 velope (1, Fig. 86) 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 the es- 

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

 and medullary sheaths are frequently absent. At intervals 

 of about ^g- inch along the fibre are found nuclei. These 

 are indications of the primitive cells which by their elon- 

 gation, fusion, and other modifications have built up the 

 nerve-fibre. In the course of a nerve-trunk its fibres rarely 

 divide; when a branch is given off some fibres merely sepa- 

 rate from the rest, much as a skein of silk might be sepa- 

 rated at one end into smaller bundles containing fewer 

 threads. 



Gray Nerve-Fibres have no medullary sheath, and con- 

 sist merely of an axis cylinder and primitive sheath. Gray 



Is each nerve-fibre continuous from centre to end? Point out 

 nerve-fibres three or four feet long. "What is the appearance under 

 the microscope of a quite fresh white nerve-fibre? How does it soon 

 alter? Name the layers then seen and state their relative positions. 

 Which is the essential part of the nerve-fibre? Give a reason for 

 your statement. What are found at intervals along the nerve-fibre? 

 What do they indicate? 



occurs when a nerve-trunk branches ? 



