404 SPECIAL HISTOLOGY. 



PERIPHERAL NERVOUS SYSTEM. 



119. Spinal nerves. The thirty-one pairs of nerves springing 

 from the spinal cord, arise, with few exceptions, by anterior and pos- 

 terior roots. Receiving a delicate tunic from the pia mater, they con- 

 verge, and are continued across the subarachnoid space, to perforate, 

 independently of each other, the arachnoid and dura mater, from the 

 latter of which they obtain a firmer coat. Proceeding further, the pos- 

 terior 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 ganglionie 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 man- 

 ner 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 connected with the neighboring 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 common trunk, it is seen that 

 the motor twigs contain principally thick fibres, and those destined for 

 the integument and other organs ab.ove 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 J-2 lines, found principally 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 aril the anterior 

 and posterior roots on the left side in a male and female body, in the 

 " Verh. d. Wurzb. phys. med.," Gesellsch. 1850, Heft II. and the 

 transverse sectional areas deduced from these observations are given in 



