CHAP, i.] THE SPINAL CORD. 867 



of posterior root, entering grey matter from external posterior column, x. grey 

 matter of posterior horn. Sp. a. bundles of fibres belonging to the spinal 

 accessory nerve; in the lateral reticular formation they are seen cut trans- 

 versely. l>. is a natural septum of connective tissue marking out the cerebellar 

 tract C. T. from the crossed pyramidal tract C.P.T. z. *. zona spongiosa. 

 2 a, /3, 7, lateral cells of the anterior horn. 5. Cells in the region of the lateral 

 reticular formation. The other letters of reference are the same as in Fig. 96. 



median group. It appears also in the thoracic region (Fig. 96, 1) ; 

 indeed the question arises whether all the cells of the anterior 

 horn in this region do not belong to this op*oup. The other 

 cells so conspicuous in the lumbar and cervical enlargements, 

 and therefore probably in some way associated with the limbs, 

 may be spoken of as forming altogether a lateral group ; but we 

 may, though with some uncertainty, subdivide them into two or 

 three groups. Thus in the lumbar region a group of cells (Fig. 

 99, 27) lying near the lateral margin of the more dorsal part or 

 base of the horn may be distinguished, as a lateral subgroup, from 

 the cells occupying the ventral lateral corner of the horn and 

 forming a ventral or anterior subgroup (Fig. 99, 2a); and the 

 same distinction, though with less success, may be made in the 

 cervical region (Fig. 98). Further, we may perhaps in both 

 regions distinguish a group of cells placed more in the very 

 middle of the horn as a central subgroup (Figs. 98, 99, 2/8). 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 represented. 



In the thoracic region a group of rather smaller cells is seen 

 at the base of the anterior horn, near to the junction with the 

 isthmus (Fig. 96, 7). In the cervical and lumbar region these cells 

 are very scanty (Figs. 98, 99, 7). 



The cells of the posterior horn contrast strongly with those of 

 the anterior horn in being few, and for the most part small. They 

 are branched ; and though we have reason to believe that, like the 

 cells of the anterior horn, they possess each an axis-cylinder 

 process, this is not easily determined by actual observation ; the 

 processes do not run out to join the posterior root as do the corre- 

 sponding processes in the anterior horn and therefore are not so 

 readily seen. These cells occur in all regions of the cord, and appear 

 to be arranged into two more groups. The lateral margin of the 

 posterior horn, at about the middle or neck of the horn, is along 

 the whole length of the cord, but especially in the cervical region, 

 much broken up by bundles of fibres passing in various directions 

 and forming an open network, called the lateral reticular formation 

 (Figs. 98, 99, r. f. lat.). In all regions of the cord a number of 

 cells are found associated with this reticular formation, forming 

 the group of the lateral reticular formation (Figs. 98, 99, 5). In 

 all regions of the cord also a group of cells (Figs. 96, 98, 99, 6) 



