460 SPECIAL HISTOLOGY. 



arachnoid and dura mater, from the latter of which they obtain 

 a firmer coat. Proceeding further, the posterior root forms 

 its ganglion, by the deposition around and among its nerve- 

 fibres, of ganglion- cells, which, to all appearance, give origin 

 to special nerve-tubes, the ganglionic fibres of the spinal-nerves, 

 each for the most part arising from a cell, and which have no 

 further connection with the fibres of the posterior root passing 

 through the ganglion, than that, in their invariably peripheral 

 course, they are in apposition, and intermingled with the 

 latter. The motor root never acquires ganglion-cells, merely 

 passing by the ganglion, in more or less close apposition with 

 it. Beyond the ganglion, the two roots are united in such a 

 manner that their elements are very intimately commingled, 

 and a common nervous trunk formed, containing in all its 

 divisions sensitive and motor elements. It is usually con- 

 nected with the neighbouring nerves above and below it, in 

 the formation of the well-known plexuses, afterwards giving 

 off its terminal branches to the muscles, integument, vessels 

 of the trunk and extremities, articular capsules, tendons, and 

 bones. As in the roots, so also in the branches of the com- 

 mon trunk, it is seen that the motor twigs contain principally 

 thick fibres, and those destined for the integument and other 

 organs above named, finer ones; ultimately, however, in the 

 terminal ramifications, all the fibres are of uniform size. The 

 fibres of all the spinal nerves appear to run quite distinct 

 from each other, and to present no divisions in the trunks 

 and branches, whilst, in the terminal ramifications of them, 

 divisions frequently occur, and, at all events in certain animals 

 (Mouse, batrachian larva), also reticular anastomoses. They 

 terminate either in loops, or in free prolongations, the latter 

 being the case, particularly, in the Pacinian bodies, which are 

 structures composed of numerous concentric capsules separated 

 by fluid, of an oval form, and measuring \ — 2'", found prin- 

 cipally in the hand and foot, and which usually contain the 

 termination of a nerve-fibre. 



[In the first and last pairs of spinal nerves occasionally only 

 a single root can be perceived, in the former case the motor, 

 and in the latter the sensitive. I have communicated the 

 diameters of all the anterior and posterior roots on the left 



