THE NERVOUS SYSTEM. 



69 



which connect the various centres, and are distributed in the several parts 

 of the body. 



NERVE FIBRES. 



Structure. Each nerve-trunk is composed of a variable number of 

 different-sized bundles (funiculi) of nerve-fibres which have a special 

 .sheath (perineurium or neurilemma). The funiculi are enclosed in a firm 



FIG. 302. Transverse section of the sciatic nerve of a cat X 100. It consists of bundles (Funiculi) 

 of nerve-fibres ensheathed in a fibrous supporting capsule, epineurium, A: each bundle has a special 

 sheath (not sufficiently worked out from the epineurium in the figure) or perineurium B; the nerve- 

 fibres N/ are separated from one another by endoneurium; L, lymph spaces; A r, artery: V, vein; 

 F,fat. (V.D.Harris.) 



fibrous sheath (epineurium)', this sheath also sends in processes of connec- 

 tive tissue which connect the bundles together. In the funiculi between 

 the fibres is a delicate supporting tissue (the endoneurium). 



There are numerous lymph -spaces both beneath the connective tissue 

 investing individual nerve- fibres, and also beneath that which surrounds 

 the funiculi. 



Varieties. In most nerves, two kinds of fibres are mingled; those 

 of one kind being most numerous in, and characteristic of, nerves of. the 

 Cerebro-spinal system; those of the other, most numerous in nerves of the 

 Sympathetic system. These are called (A) medullated or white fibres, 

 and (B) non-medullated or grey fibres. 



(A) Medullated Fibres. Each medullated nerve-fibre is made up 

 of the following parts: (1.) Primitive nerve sheath, or nucleated sheath 

 of Schwann. (2) Medullary sheath, or white substance of Schwann. 

 (3) Axis-cylinder, primitive band, axis band, or axial fibre. 



Although these parts can be made out in nerves examined some time 

 after death, in a recent specimen the contents of the sheath appear to be 

 homogeneous. But by degrees they undergo changes which show them 

 to be composed of two different materials. The internal or central part, 



