364 NERVE CELLS NERVE GRANULES. 



from those of Lie ganglia on the posterior roots of the cerebro-spinal 

 nerves, and from the nerve-cells of the ganglia of the sympathetic system. 



3. The NERVE-CELLS are spherical or oval, or polyhedral in shape, of a 

 reddish grey colour, and between 3^ and JJ S -Q of an inch in diameter. 

 Each cell is composed of a capsular sheath, and contains in its interior a 

 reddish-grey granular substance, and one or more nuclei and nucleoli, 

 the nucleus being attached to the internal surface of the sheath. The 

 sheath of the nerve-cell is constructed of three layers, the outermost con- 

 sisting of fine granular corpuscles, the middle of nucleated cells of an 

 oblong figure, and the internal of concentric lamellae of delicate cylindrical 

 filaments. Dispersed through the substance of the cell are a greater or less 

 number of pigment-granules, some being in the interior, and some in the 

 sheath. Nerve-cells are found in the grey substance of the brain and spinal 

 cord, in the ganglia of the cerebro-spinal nerves, and in the sympathetic 

 ganglia and nerves. The nerve-cells of the grey substance of the brain 

 are often very irregular in shape, and they are also remarkable for their 



oftness and for the thinness of their sheath. It is the pigment-granules, 

 contained in the nerve-cells and nerve-granules, that give the peculiar tint 

 of colour to the grey and dark substance of the cerebro-spinal axis. 



In the grey substance of the brain the nerve-cells may be seen in 

 various stages of progressive development ; thus, near the periphery, they 

 are minute, spherical or oval, nucleated cells dispersed through a matrix 

 of granular substance ; more deeply, the granular substance is collected 

 around the cells, and forms an exterior coat ; while, at a greater depth, 

 the cells have attained the size and the laminated sheaths of the fully 

 formed nerve-cells. Nerve-cells offer many peculiarities in respect of 

 number and arrangement in different parts of the nervous system. 



From the periphery of the nerve-cells one or more delicate thread-like 

 processes, between 7^ and T sJos f an mcn ^ n diameter, are given off 

 These are the origins of the grey nerve-fibres. 



4. NERVE GRANULES present the three forms of, minute homogeneous 

 particles, aggregated particles, and nucleated corpuscles, varying in size 

 between 5^0 and y^ of an inch in diameter. They serve as the bond 

 of connexion between the fibres and cells of the brain and spinal cord, 

 particularly in the grey substance ; and enter also into the composition of 

 the various ganglia. Like nerve-cells, nerve-granules contain, and have 

 intermingled with them, a variable number of pigment-granules. 



In the construction of a nerve the nerve-fibres are collected into small 

 fasciculi, each fasciculus being invested by a distinct neurilemma. These 

 fasciculi, again, are collected into bundles, forming larger fasciculi, which 

 have also a separate neurilemma ; and a bundle of the larger fasciculi, 

 enclosed in a sheath or neurilemma of white fibrous tissue, constitutes a 

 nerve The neurilemma of the smaller fasciculi is smooth and semitrans- 

 parent, and remarkable for its satiny polish, an appearance which is due 

 to the longitudinal arrangement of the undulating fasciculi of fibrous tissue 

 of which it is composed. Another character which the smaller nervous 

 fasciculi possess, is that of being crossed by oblique or transverse lines, 

 which are produced, in all probability, by the wrinkling of the neurilemma. 



In the fasciculi of grey fibres the tendency to wrinkle exists in the lon- 

 gitudinal direction, and the neurilemma is composed of an inner layer of 

 circular filaments as well as an outer layer of the longitudinal filaments of 

 fibrous tissue 



