The Lower Portion of the Human Rratn-Stem. 11 



a division of the anterior liorn into two cell-columns suggests the embryo- 

 logioal continuation of the anterior columns cephalad, to form the mesial 

 and lateral groups of motor nuclei. 



From the mesial view (figure 4) the anterior column shows, in its cervical 

 portion, a smooth mesial surface, which gradually merges into the central 

 gray matter of the cord. Throughout the extent of the decussatio pyram- 

 idum, the ventral jiortion of the mesial surface is smooth, but the dorsal 

 portion is rougliened by numerous dorsal and mesial spurs. Above the 

 crossing, the mesial surface fuses gradually into the ventrally projecting 

 masses of formatio reticularis. The formatio shows a fairly smooth surface, 

 interrupted by a fiber tract in the caudal i)ortion. This intermission of th(> 

 gray matter of the formatio on the mesial surface corresponds to the inter- 

 mission on the lateral surface, so that on mesial view the gray matter of 

 the formatio shows a hole through it. This ventrally projecting formatio 

 really consists of two ]:)lates, a lateral and a mesial, which fu.se above and 

 below, but in their middle contain a considerable fiber tract. The ])lates are 

 practically hber-free. 



The cervical portion of the anterior column and the ])art ventral to the 

 decussation exhibit a dorsal surface. In the cervical cord the dorsal lateral 

 l^order is shari)ly converted into an irregular concavity which forms the 

 dorsal surfaces throughout this extent (figure 8). This is continued mesially 

 into a well-dehned lateral ridge which projects outward from the central 

 gray matter. In the region of the decussation of the pyramids the dorsal 

 surface is marked by irregularities and shows many dorsal and mesial spurs 

 projecting between the crossing bundles of the jiyramids. This surface is 

 eliminated as the column fuses with the formatio reticularis. 



As will be seen in figure 2, the anterior column, after its fusion with the 

 formatio reticularis, is abruptly and arbitrarily ended in the model. This 

 was done for several i:)iu'poses, particularly as it was thought ath'isable to 

 show the olivary complex comj^letely sei)arated from the formatio reticularis 

 on the one side (left) and developing out of it on the other (right side of 

 model, figure 1). 



SUBSTANTIA GRISEA CENTRALIS AND FORMATIO RETICULARIS. 



The two sides of this model were reconstructed for different piu'poses, 

 so that on the right side the reconstruction should deal only with the unsep- 

 arated masses of gray matter, outlines being drawn around the extreme 

 portions of the gray matter present in the series of transverse sections. 

 Hence, on this right side, the substantia grisea centralis, that indifferent 

 mass of gray matter about the central canal of the spinal cord (figures cS and 

 9), is modeled together with the extreme portions, such as the nucleus fasci- 

 culi gracilis, the nucleus fasciculi cuneati, the substantia gelatinosa (figures 

 10 and 11), etc. Tracing this gray matter about the canal cephalad, we 

 find it merging with the formatio reticularis of the medulla and pons, but 



