GENERAL STRUCTURE OF THE NERVOUS SYSTEM. 343 



The function of the nervous system is therefore to associate the dif- 

 ferent parts of the body in such a manner, that stimulus applied to 

 one organ may excite the activity of another. 



The instances of this action are almost as numerous as the vital 

 phenomena. The light falling upon the retina produces contraction 

 of the pupil. Introduction of food into the stomach causes a discharge 

 of bile from the gall-bladder. Alimentary substances, in contact with 

 the mucous membrane of the intestine, excite the peristaltic action 

 of its muscular coat; and the presence of a foetus in the uterus is 

 accompanied by increased growth of the mammary glands. Every 

 organ is subservient, in the manifestation of its activity, to influences 

 derived from other parts through the nervous system. 



In the nervous system there are two kinds of anatomical elements ; 

 namely, nerve fibres and nerve cells. The nerve fibres are the charac- 

 teristic constituents of the "white substance," forming the mass of 

 the nerves and their ramifications, the external portion of the spinal 

 cord, and much of the internal parts of the brain. The nerve cells are 

 found in the "gray substance," which constitutes the external or con- 

 voluted layer of the brain, as well as various internal deposits near its 

 base, the central portions of the spinal cord, and many small detached 

 masses, or ganglia, in different parts of the body. 



Nerve Fibres. 



The nerve fibres are cylindrical filaments, arranged in bundles or 

 tracts, for the most part parallel with each other. Their diameter 

 varies considerably, even in the same locality ; some of the fibres in a 

 single bundle being 10, 15, or 18 micro-millimetres in diameter, while 

 others are not more than 2.5 mmm. Their average size also varies in 

 different parts of the nervous system. The larger fibres are found in 

 the peripheral trunks and branches of the nerves, where they have an 

 average diameter of 12.5 mrnm. ; in the white substance of the brain 

 and spinal cord their average diameter is 5 mmm., and in the gray 

 substance it is reduced to 2 mmm. Certain portions of the nervous 

 system are distinguished by the comparative abundance of their larger 

 or smaller fibres. Thus in the cutaneous nerves of man, according to 

 Bidder, Yolkmanu, and Kollikcr, the larger and smaller fibres are in 

 about equal quantity, while in the muscular nerves the larger fibres are 

 three times as abundant as the smaller. In the nerves of bony tissue 

 the number of small fibres is double that of the large ones; and in the 

 gray substance of the cerebral hemispheres they all belong to the 

 smaller variety, none being larger than 6 of Y mmm. in diameter. 

 The nerve fibres in the same bundle or tract may increase or diminish 

 in size at different parts of their course ; as Kolliker has shown that 

 the fibres of the posterior roots of the spinal nerves, in passing to the 

 gray substance of the cord, are reduced in average diameter from 1 

 to 5 mmm., and those of the white substance of the cerebral hemi- 



