538 THE SPINAL COED. 



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. 119, 7). 

 In the cervical and lumbar region these cells are very scanty (Figs 121 

 122, 7). 



Tfie cells of the posterior horn contrast strongly with those of the ante- 

 rior 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 deter- 

 mined by actual observation ; the processes do not run out to join the poste- 

 rior root, as do the corresponding processes in the anterior horn, and there- 

 fore are not so readily seen. These cells occur in all regions of the cord, 

 and appear to be arranged in two or 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 retieular formation (Figs. 121, 122, r.f. 

 lat.). In all regions of the cord a number of cells are found associated 

 with this retieular formation, forming the group of the lateral retieular for- 

 mation (Figs. 121, 122, 5). In all regions of the cord, also a group of cells 

 (Figs. 119, 121, 122, 6) is found in that part of the horn where, a little 

 ventral to the substance of Rolando, the uniform field of gray matter is 

 broken up into a kind of network by a number of bundles of white fibres 

 running in various directions. The network has also been called a retieu- 

 lar formation, and has received the name of posterior retieular formation 

 (Figs. 121, 122, r.f. p.) to distinguish it from the lateral retieular forma- 

 tion just mentioned ; the two, however, in some regions (see Fig. 119) join 

 each other, and thus cut off a ventral portion of the posterior horn con- 

 taining nerve-cells from a dorsal portion, x in Figs. 121, 122, in which no 

 obvious or conspicuous nerve-cells are present. 



The groups of cells just mentioned, with the restrictions and modifications 

 spoken of, occur along the whole length of the cord ; but the group of cells 

 to which we must now call attention is almost confined to a special region 

 of the cord, or at least is but feebly represented elsewhere. In the thoracic 

 region, especially in the lower thoracic region (we shall return to the limits 

 of the group later on), at the base of the posterior horn (Fig. 119, 3), just 

 ventral to the curve formed by the posterior gray commissure as this bends 

 dorsally to join the posterior horn, is seen on each side of the cord a con- 

 spicuous group of cells known as Clarke's column, or the posterior vesicular 

 column or vesicular cylinder. The cells composing this group, though vary- 

 ing in size at different levels, are rather large cells, and are for the most 

 part fusiform, with their long axis placed lengthways along the cord, so 

 that in transverse sections they often appear to have a rather small round 

 body. They are surrounded by, and as it were imbedded in, a mass of fine 

 fibres, the area of which is indicated by a dotted line in Fig. 118. 



Also conspicuous in the thoracic region is another group of cells lying 

 on the outer side of the middle of the gray matter at about the junction of 

 the anterior and posterior horns. This is known as the intermedio-lateral 

 tract, and is sometimes called the lateral horn (Fig. 119, 4). The cells com- 

 posing it are somewhat small spindle-shaped cells with their long axis 

 placed transversely. The group is conspicuous, as we have said, in the 

 thoracic region ; it may be recognized in the lumbar region (Fig. 121, 4), 

 but in the cervical region becomes confused with the most dorsally placed 

 or lateral sub-group of the anterior horn. We shall, however, have to re- 

 turn to these groups of cells when we come to speak of the differences 

 between the spinal regions of the cord. 



