72 



THE HUMAN MECHANISM 



consists of a central threadlike core surrounded by a fatty 

 sheath, the latter being, therefore, shaped like a hollow 

 cylinder, which, however, is interrupted at intervals of 

 about one millimeter, and both of these are enveloped in a 

 delicate membrane comparable to the sarcolemma of the muscle 

 fiber. Such fibers are from about -5-^5-5- to y-^Q- of an inch 

 in diameter (compare the diameter of a muscle fiber, p. 34). 



There are, however, nerve fibers 

 which have no fatty sheath, and 

 others which are destitute of mem- 

 brane. The essential part of the 

 fiber is the threadlike portion in 

 the center; this is never absent 

 from nerves and is known as the 

 axon, or axis cylinder. 



3. The axon of a nerve fiber is 

 a branch of a nerve cell. By suit- 

 able methods these axons may be 

 traced along the nerve of which 

 they form part and even into the 

 brain and spinal cord; it is then 

 found that they pursue an uninter- 

 rupted course and ultimately become continuous with the 

 cytoplasm of a nerve cell. Nerve cells are found in the brain, 

 in the spinal cord, in enlargements (ganglia) on certain 

 nerves, and even alone in the connective tissue of many or- 

 gans of the body, as the heart, the intestine, etc. By far the 

 greater number are in the brain and spinal cord, and in some 

 cases the axons to which they give rise are of very consider- 

 able length; those of the muscles of the foot, for example, 

 reach from cells in the sacral region of the spinal cord to the 

 extremity of the foot. Such fibers would be over a yard long 

 and less than y^o" ^ an mc ^ w ide, an( ^ we mav regard the 

 cell whose main portion is in the sacral cord as sending out 

 a branch, or process, from this region to the foot. 



FIG. 40. Four nerve fibers 

 (highly magnified) 



R, node of Ranvier at which 

 the fatty sheath is discon- 

 tinuous 



