GREY NERVE FIBRES. 363 



merest lamina in the fish, have become the largest portion of the brain in 

 man. And the fifth pair (olfactory lobes), so large in the lowest forms, 

 have dwindled into the olfactory bulbs of man. 



The microscopic elements of the nervous system, are, white nerve- 

 fibres, grey nerve-fibres, nerve-cells, and nerve-granules. 



1. WHITE NERVE FIBRES are the chief con- 

 stituent of the brain, the spinal cord, and the Fi s- 168 * 

 cerebro-spinal nerves, and they also enter into 



the composition of the sympathetic nerve. 

 They present some variety of size in different 

 parts of the nervous system, measuring in the 

 brain between B ^ 05 and T7 J^o f an mc h m 

 diameter, and in the cerebro-spinal nerves be- 

 tween ^^(j and 3 o'ac> f an mcn - As a general 

 rule, the white nerve-fibres are largest in the 

 nerves, smaller where they enter the cerebro- 

 spinal mass, and smallest at their termination, 

 centrally, in the grey substance of the surface of the brain, and, periphe- 

 rally, in the tissues of the body. In structure, each white nerve-fibre is 

 composed of a transparent and structureless cylindrical tubule or sheath 

 (vagina medullaris), and of an axis-cylinder filled with an opalescent, 

 colourless, oil-like fluid (neurine), which coagulates after death, and then 

 resembles a white, opaque, and curd-like matter. The vagina medullaris 

 possesses somewhat less than one-third the thickness of the entire fibre, 

 and gives to the latter, when examined with the microscope, the aspect 

 of a double cylinder, an appearance which is characteristic of the white 

 nerve-fibre. It is thickest in the fibres of the spinal nerves, and thinnest 

 in those of the spinal cord, brain, and nerves of special sense. Hence in 

 these latter, the sheath, when pressed or stretched, is apt to assume a 

 varicose appearance, and the contained substance to accumulate in small 

 separate masses. White nerve-fibres terminate, both at the surface of the 

 body, in the various internal organs, and in the substance of the cerebro- 

 spinal axis, by forming loops. 



2. GREY NERVE FIBRES (fine nerve-fibres, gelatinous fibres, sympathetic 

 fibres) are about one-half or one-third less in diameter (3^3 to 5^0 of an 

 inch, Henle) than the \vhite fibres. They are less transparent, have no 

 appearance of being composed of a double cylinder, and their sheath is 

 less easily distinguishable from its contents. In structure, they consist of 

 a thin and finely granulated sheath, filled with granular substance, and, 

 when collected into a fasciculus, have a yellowish grey tint. The grey 

 nerve-fibres are abundant in, and are indeed the chief constituent of, the 

 sympathetic system. They are also present in the cerebro-spinal nerves, 

 and, most numerously, in the nerves of sensation. They take their origin 

 from the nerve-cells of the grey substance of the brain and spinal cord, 



* Minute structure of nerve. 1. The mode of termination of white nerve-fibres in 

 loops ; three of these loops are simple, the fourth is convoluted. The latter is found in 

 situations where an exalted degree of sensation exists. 2. A white nerve-fibre from 

 the brain, showing the varicose appearance produced by traction or pressure. 3. A 

 white nerve-fibre enlarged to show its structure, namely, a tubular envelope, and a 

 contained substance, neurine. 4. A nerve-cell, showing its composition of a granular 

 looking capsule and granular contents. 5. Its nucleus containing a nucleolus. 6. A 

 nerve-cell from which several processes are given off. It contains, like he preceding, 

 a nucleolated nucleus. 7. Nerve-granules. 



