66 A Reconstruction of the Nuclear Masses in 



the same histological structure as are the pontine nuclei, and at the lower end 

 of the pons the arcuate nuclear material insensibly passes into the pontine 

 masses. It is believed, from consideration of these factors, that any descrip- 

 tion of these nuclei should deal with the three nuclear masses as integral parts 

 of one main nuclear mass. This plan will be followed in the description, the 

 arcuate nuclei and the corpus ponto-bulbare being considered first, and then 

 the pontine nuclei as the cephalic portion of these integral parts. 



Inspection of a number of brain-stems cut in serial sections will lead 

 to the conclusion that marked differences exist in the amount of nuclear 

 material comprised in the arcuate nucleus and in the corpus ponto-bulbare. 

 Some of the series will show great amounts of this gray matter scattered 

 along the course of the fibra? arciformes externtB ventrales with large redun- 

 dant arcuate nuclei, while in other brain-stems the amount of these peripheral 

 cell-masses is meager and very small. With such marked diversity in the 

 extent of the nuclear material along the ventral surface of the medulla, each 

 reconstruction must be considered wholly from the standpoint of the presen- 

 tation of the nuclear material in that particular brain-stem. Similar differ- 

 ences exist in the amount of the cellular material comprising the corpus 

 ponto-bulbare. The adult brain-stem, which has here been reconstructed, 

 is very poor in the nuclear matter comprising both of these cell-masses and 

 the resulting model of these two cell-columns must be taken as indicative 

 of the extent and course of the masses rather than as the morphological 

 form for every brain-stem. Similar differences, of course, exist in all nuclear 

 masses, but these differences are not in the main sufficient to change appreci- 

 ably the morphology. 



NUCLEI ARCIFORMES. 



The nuclei arciformes in this reconstruction exhibit considerable differ- 

 ences on the two sides of the model, as can be seen from figure 1. Neither 

 of them is a continuous cell-mass, but both are interrupted about the middle 

 of their extent. Both begin at approximately the same level caudally, just 

 caudal to the inferior termination of the nucleus olivaris accessorius medialis. 

 Both finally terminate by merging with the pontine nuclei at the caudal 

 extremity of the pons, their extent being about coincident witli that of the 

 inferior olive. 



The arcuate nucleus on the left side begins caudally as a flat, rather 

 thin sheet of cells, which is placed in the midst of the ventral external arcuate 

 fibers, ventral to the pyramids. With the smallest diameter dorso-ventral, 

 the column can be traced cephalad (figure 2) as a continuous cell-mass to 

 slightly superior to the point of radiation on the vcnti-al leaf of the olive. 

 In this extent of the lower continuous mass, the column shows a lateral 

 convexity, so that even the upper end of this first portion of tlic nucleus is 

 placed lateral to the inferior end. Two marked lateral notches are seen on 

 the mesial border in its convexity. Just caudal to the inferior of these 

 notches is a well-defined lateral projection (figure 1). A much smaller 



