94 HAND-BOOK OF PHYSIOLOGY. 



directly, but only through the intermediation of Gerlach's nerve-network, 

 in which their branching processes lose themselves. 



Spinal Nerves. The spinal nerves consist of thirty-one pairs, 

 issuing from the sides of the whole length of the cord, their number corre- 

 sponding with the intervertebral foramina through which they pass. 

 Each nerve arises by two roots, an anterior and posterior, the latter being 

 the larger. The roots emerge through separate apertures of the sheath of 

 dura mater surrounding the cord; and directly after their emergence, where 

 the roots lie in the intervertebral foramen, a ganglion is found on the pos- 

 terior root. The anterior root lies in contact with the anterior surface of 

 the ganglion, but none of its fibres intermingle with those in the gan- 

 glion (5, Fig. 316). But immediately beyond the ganglion the two roots 

 coalesce, and by the mingling of their fibres form a compound or mixed 

 spinal nerve, which, after issuing from the intervertebral canal, divides 

 into an anterior and posterior branch, each containing fibres from both 

 the roots (Fig. 316). 



The anterior root of each spinal nerve arises by numerous separate and 

 converging bundles from the anterior column of the cord; the posterior 

 root by more numerous parallel bundles, from the posterior column, or, 

 rather, from the posterior part of the lateral column (Fig. 318), for if a fis- 

 sure be directed inward from the groove between the middle and pos- 

 terior columns, the posterior roots will remain attached to the former. 

 The anterior roots of each spinal nerve consist of centrifugal fibres; the 

 posterior as exclusively of centripetal fibres. 



Course of the Fibres of the Spinal Nerves. (a) The Anterior 

 roots enter the cord in several bundles which may be called: (1) Inter- 

 nal; (2) Middle; (3) External; all being more or less connected with the 

 groups of multipolar cells in the anterior cornua. 1. The internal fibres 

 are partly connected with internal group of nerve cells of anterior cornu 

 of the same side; but some fibres pass over, through anterior commissure, 

 to end in the anterior cornu of opposite side, probably in internal group 

 of cells. 2. The middle fibres are partly in connection with the lateral 

 group of cells in anterior cornu, and in part, pass backward to posterior 

 cornu, having no connection with cells. 3. 'The external fibres are partly 

 in connection with the lateral group of cells in the anterior cornu, but 

 some fibres proceed direct into the lateral column without connection with 

 cells and pass upward in it. 



() The Posterior roots enter the posterior cornu in two chief bundles, 

 either at the tip, through or round the substantia gelatinosa, or by the 

 inner side. The former enter the grey matter at once, and as a rule, turn 

 upward or downward for a certain distance and then pass horizontally, 

 some fibres reach the anterior cornua, passing at once horizontally; and 

 the others, the opposite side, through the posterior grey commissure. Of 

 those which enter by the inner side of the cornua the majority pass up 



