THE CEREBKO-SPINAL NERVOUS SYSTEM. 477 



the tip of the cornu in the middle line sometimes, as in the lumbar en- 

 largement, there is a third group more posterior. The cells of the an- 

 terior group are the largest. Into many of these cells the fibres of the 

 anterior motor nerve-roots can be distinctly traced, (b) In the tractus 

 intermedio-lateralis. A group of nerve-cells midway between the ante- 

 rior and posterior cornua, near the external surface of the gray matter. 

 It is especially developed in the dorsal, and also in the upper cervical re- 

 gion, (c) In the posterior vesicular columns of Lockhart Clarke. These 

 are found in the posterior cornua of gray matter towards the inner sur- 

 face, extending from the cervical enlargement to the third lumbar nerves 

 (Fig. 332, c). (d) Smaller cells are scattered throughout the gray mat- 

 ter, but are found chiefly at the tip (caput cornu) of posterior cornu, in 

 a finely granular basis, and also among the posterior root fibres (substan- 

 tia gelatinosa cinerea of Eolando). 



The anterior nerve-cells are connected by their processes immediately 

 with the axis-cylinders of the fibres of the anterior or motor nerve-roots: 

 whereas the nerve-cells of the posterior roots are connected with nerve- 

 fibres, not directly, but only through the intermediation of G-erlach's 

 nerve-network, in which their branching processes lose themselves. 



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

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

 sponding with the inter vertebral 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 posterior root. The anterior root lies in contact with the anterior 

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

 the ganglion (5, Fig. 330). 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, 

 gives off anterior and posterior or ventral and dorsal branches, each 

 containing fibres from both the roots (Fig. 330), as well as a third or 

 visceral branch, ramus communicans, to the sympathetic. 



The anterior root of each spinal nerve arises by numerous separate 

 and converging bundles from the anterior column of the cord; the pos- 

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

 or, rather, from the posterior part of the lateral column (Fig. 330), for 

 if a fissure be directed inwards from the groove between the middle and 

 posterior columns, the posterior roots will remain attached to the for- 

 mer. 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 Nerve-Roots, (a) The An- 

 terior roots enter the cord in several bundles, which may be called: 



