536 THE SPINAL CORD. 



be nerve-cells, form, as we have seen, only a part of the gray matter, and in 

 some parts of the cord, in the thoracic region for instance, are so sparse that 

 in a section of the spinal cord iu this region thin enough to show its histo- 

 logical features satisfactorily, the bodies of a few only of such cells are 

 visible (Fig. 119) ; the greater part of the gray matter consists not of the 

 bodies of conspicuous nerve-cells, but of a mass of fibres and fibrils passing 

 apparently in all directions. In the cervical (Fig. 121), and especially in 

 the lumbar (Fig. 122) regions, the nerve-cells are both absolutely and rela- 

 tively more abundant ; but even in a section taken from the lumbar region 

 the nerve-ce'lls, all put together, form the smaller part of the whole area of 

 gray matter. Moreover, in respect of the number of cells, all the sections 

 of even the same region of the cord are not alike. Seeing that the cord may 

 be considered as growing out of the fusion of a series of paired ganglia, each 

 ganglion corresponding to a nerve (c/. 92), we may fairly expect to find the 

 fusion not complete, so that the nerve-cells would appear more numerous 

 opposite a nerve than in the middle between two nerves. In some of the 

 lower animals this arrangement is most obvious, and there are some reasons 

 for thinking that even in man the nerve-cells are metamerically increased at 

 the level of each nerve. 



Even when casually observed, it is obvious that the nerve-cells are not 

 scattered in a wholly irregular manner throughout the gray matter, being, 

 for instance, much more conspicuous in the anterior horn than elsewhere ; 

 and more careful observation allows us to arrange them to a certain extent 

 in groups. 



The cells of the anterior horn are for the most part large and conspicuous, 

 67/J- to 135/x in diameter, branch out in various directions, and present an 

 irregular outline in sections taken in different planes. We have reason to 

 think that every one of them possesses an axis-cylinder process, which, in 

 the case at all events of most of the cells, passing out of the gray matter 

 becomes a fibre of the adjacent anterior root. They are obvious and con- 

 spicuous in all regions of the cord, though much more numerous and indi- 

 vidually larger in the cervical and lumbar enlargements than in the thoracic 

 region. We may further, with greater or less success, divide them into 

 separate groups. 



In the cervical and lumbar regions a fairly distinct group of cells is seen 

 lying on the median side of the gray matter close to the anterior column 

 (Figs. 121, 122, 1). This may be called the median group. It appears also 

 in the thoracic region (Fig. 119, 1) ; indeed, the question arises whether all 

 the cells of the anterior horn in this region do not belong to this group. The 

 other cells so conspicuous in the lumbar and cervical enlargements, and there- 

 fore probably in some way associated with the limbs, may be spoken of as 

 forming altogether a lateral group ; but we may, though with some uncer- 

 tainty, subdivide them into two or three groups. Thus in the lumbar region 

 a group of cells (Fig. 122, 2 ^) lying near the lateral margin of the more 

 dorsal part or base of the horn may be distinguished, as a lateral sub-group, 

 from the cells occupying the ventral lateral corner of the horn and forming 

 a ventral or anterior sub-group (Fig. 122, 2 oc) ; and the same distinction, 

 though with less success, may be made in the cervical region (Fig. 121). 

 Further, we may perhaps in both regions distinguish a group of cells placed 

 more in the very middle of the horn as a central sub -group (Figs. 121, 122, 

 2 /?). But, in all cases, the separation of these cells, which we have spoken 

 of as a whole as lateral cells, into minor groups, is far less distinct than the 

 separation of the median group from these lateral cells, especially if we 

 admit that in the thoracic region the median group is alone clearly repre- 

 sented. 



