CHAP, i.] THE SPINAL CORD. 869 



reticular formation, and has received the name of posterior reticular 

 formation (Figs. 98, 99, r. /. p.) to distinguish it from the lateral 

 reticular formation just mentioned ; the two however in some 

 regions (see Fig. 96) join each other, and thus cut off a ventral 

 portion of the posterior horn containing nerve-cells from a dorsal 

 portion, x in Figs. 98, 99, 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. 96, 3) just ventral to the curve formed by the posterior grey 

 commissure as this bends dorsally to join the posterior horn, is 

 seen on each side of the cord a conspicuous group of cells known 

 as Clarke's column or the posterior vesicular column or vesicular 

 cylinder. The cells composing this group, though varying 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. 96. 



Also conspicuous in the thoracic region is another group of 

 cells lying on the outer side of the middle of the grey matter at 

 about the junction of the anterior and posterior horns. This is 

 known as the intermedia-lateral tract and is sometimes called 

 the lateral horn (Fig. 96, 4). The cells composing it are some- 

 what small spindle-shaped cells with their long axis placed trans- 

 versely. The group is conspicuous as we have said in the thoracic 

 regions ; it may be recognized in the lumbar region (Fig. 99, 4), 

 but in the cervical region becomes confused with the most dorsally 

 placed or lateral subgroup of the anterior horn. We shall however 

 have to return to these groups of cells when we come to speak; of 

 the differences between the several regions of the cord. 



567. The tracts of white matter. At first sight the white 

 matter of the cord appears to be of uniform nature. We can use 

 the nerve roots to delimitate the anterior, posterior and lateral 

 columns, but we appear to have no criteria to distinguish parts in 

 each column. In the cervical and upper thoracic regions of the 

 cord, a septum (Fig. 96, s.) in the posterior column, somewhat 

 more conspicuous than the other septa, has enabled anatomists 

 to distinguish an inner median portion, the median posterior 

 column, commonly called the poster o-median column or column of 

 Goll (Fig. 96, m. p.), from an outer lateral portion, the external 

 posterior column, commonly called the postero-external column or 



