The Lower Portion of the Hiiriuni BroiJi-Sfeii). 9 



DESCRIPTION OF TRANSITIONS FROM CORD TO MEDULLA. 

 THE ANTERIOR MOTOR COLUMN. 



The anterior motor column has been reconstructed in this mo(l(4 from 

 the upper cervical region to the point where it blends with the formatio 

 reticularis of the medulla. On the left side of the model (figure 2) the sep- 

 aration of the anterior colunm (mc) from the tegmental gray matter has 

 been shown, but on the right side the fine extensions of nuclear material 

 from the tegmental portion to the column have been included (cf. two sides 

 of figure 9), so that the course of the pyramids into the lateral columns in 

 the cord is not given. The morphology resulting from the modeling on the 

 right side will be discussed under the subdivision of the substantia grisea 

 centralis and the formatio reticularis. The j^resent portion of this communi- 

 cation treats only of the morphology of the isolated anterior column. The 

 term "anterior column" will be used throughout to designate the collection 

 of motor cells in the anterior portion of the gray matter of the sj^inal cord 

 {mc, figure 8); the term ''anterior horn" will not be used, as it does not 

 represent the continuous character of the cell-collection. 



Figiu'e 8 shows the typical transverse section of the upper cervical cord. 

 In this the anterior column api)ears on section as an irregularly triangular 

 mass of motor cells and supporting tissue. The anterior column (me), as 

 modeled, is shown in figures 1, 2, and -i, dealing here with the left side of the 

 reconstruction. It does not present any well-defined surfaces for examina- 

 tion, but is probably best described from the hiteral, ventral, and mesial views 

 a shown in the drawings. Roughly, it may be divided here into the cervical 

 portion, the jjortion isolated from the gray matter ]\v the ]\vramids, and the 

 superior portion, in which the anterior coluniii fuses with the formatio 

 reticularis. 



When viewed from the lateral surface (figiue 2), the anterior colunm in 

 its cervical portion exhibits a somewhat rounded vcntro-lateral surface, 

 which is sharply defined dorsally, but curves gradually around the ventral 

 side. Its dorsal margin is a fairly straight, sliglitly irregular projection. 

 Just ventral to this is a .small, elevated ridge which runs cephalo-caudally 

 throughout this portion. \'entral to this ridge is a very shallow longitiulinal 

 depression, anterior to which the rounding of the surface ventrally occurs. 

 The character of the column changes considerably as the region of its sepa- 

 ration by the pyramids is reached (figure 9). The dorsal margin, at the 

 caudal end of this decussatio i^yramidum (cf. figure 2), is marked by irregu- 

 larities, the most caudal of which becomes a well-defined dorsal spur. This 

 is followed, cephalad, by a convex sheet-like i)rojection dorsally, superior 

 to which is a shallow ventral dejiression. The cephalic l)order of this gradu- 

 ally inclines dorsally and then runs sharply cephalo-caudally to the suiK^rior 

 limit of tlie decussation. The lateral ritlge, noted in the cervical poi'tion of 



