NERVE-FIBRES. 



295 



cells are found in the centres only; while the fibres, of. 

 which there are two main varieties known as the white and 

 the gray, are found in both trunks and centres; the white 

 variety predominating in the cerebro-spinal nerves and in 

 the white substance of the centres, and the gray in the sym- 

 pathetic trunks and the gray portions of the central organs. 

 White Nerve-Fibres consist of extremely delicate threads, 

 about s-jjVo inch in diamete^ but frequently of a length 



FIG. 85. 



FIG. 86. 



FIG. 85. White nerve-fibres soon after removal from the body and when they 

 have acquired their double contour. 



FIG. 86. Diagram illustrating the structure of a white or medullated nerve- 

 fibre. 1, 1, primitive sheath; 2, 2, medullary sheath; 3, axis cylinder. 



Where only do we find nerve-cells? Where are nerve-fibres 

 found? How many main kinds of nerve-fibres are there? Name 

 them. Where do the white nerve-fibres predominate? Where the 

 gray? 



Describe white nerve-fibres. 



