SIZE AND STRUCTURE. MODE OF DIVISION. 631 



These fibres in man and the mammalia appear to pass through the ganglion with- 

 out union with its cells. The cells are both unipolar and bipolar, but the fibres con- 

 nected with them all pass to the periphery (Kolliker), so that beyond the ganglion 

 the posterior root of the nerve has received an additional set of fibres besides those 

 which it contains before reaching the ganglion. In fishes, on the contrary, all the 

 fibres of the posterior root are connected with the opposite extremities of the bipolar 

 cells of the ganglion. 



The anterior roots of the spinal nerves are, as will be inferred from what 

 has been already stated, the smaller of the two ; they are devoid of 

 ganglioiiic enlargement, and their fibres are collected into two bundles near 

 the intervertebral ganglion, as in the posterior root. 



Size, The roots of the upper cervical nerves are smaller than those of the 

 lower nerves, the first being much the smallest. The posterior roots of 

 these nerves exceed the anterior in size more than in the other spinal nerves, 

 and they are likewise composed of fasciculi which are considerably larger 

 than those of the anterior roots. 



The roots of the dorsal nerves, exception being made of the first, which 

 resembles the lowest cervical nerves and is associated with the in in a part of 

 its distribution, are of small size, and vary but slightly, or not at all, from 

 the second to the last. The fasciculi of both roots are thinly strewed over 

 the spinal cord, and are slender, those of the posterior exceeding in thickness 

 tho^e of the anterior root in only a small degree. 



The roots of the lower lumbar, and of the upper sacral nerves, are the 

 largest of all the spinal nerves ; those of the lowest sacral and the coccygeal 

 nerve are, on the other hand, the slenderest. All these nerves are crowded 

 together round the lower end of the corJ. Of these nerves the anterior 

 roots are the smaller, but the disproportion between the anterior and pos- 

 terior roots is not so great as in the cervical nerves. 



Length of the nerves in the spinal canal. The place at which the roots 

 of the upper cervical nerves are connected with the spinal cord being nearly 

 opposite the foramina by which they respectively leave the canal, these roots 

 are comparatively short. But the distance between the two points referred 

 to is gradually augmented from nerve to nerve downwards, so that the 

 place of origin of the lower cervical nerves is the breadth of at least one 

 vertebra, and that of the lower dorsal nerves about the breadth of two 

 vertebrae above the foramina by which they respectively emerge from the 

 canal. Moreover, as the spinal cord extends no farther than the first 

 lumbar vertebra, the length of the roots of the lumbar, sacral, and coccygeal 

 nerves increases rapidly from nerve to nerve, ami in each case may be 

 estimated by the distance of the foramen of exit from the extremity of the 

 cord. Owing to their length, and the appearance they present in connection 

 with the spinal cord, the aggregation of the roots of the nerves last referred 

 to has been named the " cauda equina." 



The direction the roots take within the canal requires brief notice. The 

 first cervical nerve is directed horizontally outwards. The roots of the 

 lower cervical and dorsal nerves at first descend over the spinal cord, held 

 in contact with it by the arachnoid, till they arrive opposite the several 

 iutervertebral foramina, where they are directed horizontally outwards. 

 The nerves of the cauda equina run in the direction of the spinal canal. 



Division of the nerves. The two roots of each of the spinal nerves unite 

 immediately beyond the ginglion, and the trunk thus formed separates, 

 as already mentioned, into two divisions, an anterior and a posterior, which 

 are called primary branches or divisions. 



In the detailed description of the spinal nerves which follows, we shall 



