CHAP, i.] THE SPINAL CORD. 865 



in B is reached by the growth of the posterior columns. From B, 

 the transition to the normal state of things as in 97, c, is a very 

 slight one. The extreme dorsal tip of the horn being of a more 

 open texture than the substance of Rolando, is sometimes called 

 the zona spongiosa. 



566. The grouping of the nerve-cells. The nerve-cells, at all 

 events the cells which are large enough to be easily and without 

 doubt recognized to be nerve-cells, form, as we have seen, only 

 a part of the grey 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 in this region thin enough to shew its histological 

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

 visible (Fig. 96) ; the greater part of the grey 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. 98) and especially in the lumbar (Fig. 99) regions 

 the nerve-cells are both absolutely and relatively more abundant ; 

 but even in a section taken from the lumbar region the nerve- 

 cells, all put together, form the smaller part of the whole area of 

 grey 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, 

 cf. 96, 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 grey 

 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 /* to 135 //- in diameter, branch out in various direc- 

 tions, 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 grey matter becomes a fibre of the 

 adjacent anterior root. They are obvious and conspicuous in all 

 regions of the cord, though much more numerous and individually 

 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 grey matter close to 

 the anterior column (Figs. 98, 99, 1). This may be called the 



552 



