366 CLASSIFICATION OF NERVES. 



colour, are invested by a smooth membranous sheath, and are composed 

 of the three essential constituents of the nervous system, namely, nerve- 

 cells, nerve-fibres, and nerve-granules. The nerve-cells of ganglia are 

 firm in structure, and have stronger investing sheaths than those of the 

 brain. From the exterior of their sheaths, filaments of fibrous tissue are 

 given off, which interlace with each other, and hold the cells together ; 

 and at the same time form an investing network around the entire gan- 

 glion, the nerve-fibres passing into and out of the ganglion through the 

 interstices of this network. Besides the sheath-filaments, certain of the 

 nerve-cells give off grey fibres, while others are free. The nerve-fibres of 

 ganglia are of the two kinds met with in the rest of the nervous system. 

 The white-fibres are derived from the cerebro-spinal axis, and enter the 

 sympathetic through the so-called roots of that nerve, namely, its commu- 

 nications with the spinal nerves. In the ganglia these white fibres sepa- 

 rate, and either pass directly onwards between the nerve-cells (traversing 

 fibres), or make a series of turns around them (winding fibres) ; in either 

 case, they collect together after a plexiform course between the nerve-cells, 

 and form fasciculi, which pass off as branches from the ganglion. The 

 grey nerve-fibres originate from certain of the nerve-cells within the gan- 

 glion as finely granular threads, and pass away in the form of fasciculi, 

 with or without association of the white fibres, to be distributed to the 

 various organs, or to traverse other ganglia previously to their distribution. 

 The Nerve-granules occupy the interstices between the nerve-cells and 

 the nerve-fibres, as in the cerebro-spinal mass. They are also continued 

 with the nerve-fibres, into some of the nerves given off by the ganglia. 

 Like the interstitial substance of the brain, the granular substance (gela- 

 tinous substance) of ganglia has intermingled with it minute cells and 

 pigment-granules. 



Nerves are divisible into two great classes ; those which proceed di- 

 rectly from the cerebro-spinal axis, the cranial and spinal nerves, and 

 constitute the system of animal life ; and those which originate from the 

 sympathetic system, or system of organic life. 



The division of nerves into cranial and spinal is purely arbitrary, and 

 depends on the circumstance of the former passing through the foramina 

 of the cranium, and the latter through those of the vertebral column. 

 With respect to origin, all the cranial nerves, with the exception of the 

 first, [olfactory,] proceed from the spinal cord, or from its immediate pro- 

 longation into the brain. The spinal nerves arise by two roots ; anterior, 

 which proceeds from the anterior segment of the spinal cord, and possesses 

 a motor function ; and posterior, which is connected with the posterior 

 segment, and bestows the faculty of sensation. The motor nerves of 

 the cranium are shown by dissection to be continuous with the motor 

 portion of the cord, and form one system with the motor roots of the 

 spinal cord ; while the nerves of sensation, always excepting the ol- 

 factory, are in like manner traced to the posterior segment of the cord, 

 and form part of the system of sensation. To these two systems a third 

 was added by Sir Charles Bell, the respiratory system, which consists of 

 nerves associated in the function of respiration, and arising from the side 

 of the upper part of the spinal cord in one continuous line, which he 

 thence named the respiratory tract. The microscope has failed in making 

 out any structural distinction between the anterior and posterior roots of 

 the spinal nerves ; but the latter are remarkable for the possession of a 



