The Lower Poiiion. of (he Hinudii Bnihi-Stem. 65 



three divisions of the main nuclear sheet. The constriction, however, 

 between the caudal and middle lobes, is further accentuated by the presence 

 of a large fenestration in the cell-sheet, made by the coursing fiber bundles. 

 The cephalic division is not large nor well marked off, Init it follows, in the 

 main, the same general direction and relationship as does the corresponding 

 mass on the opposite side. Dorsal to the middle part of the middle division 

 is a somewhat larger dorsal projection than on the opposite side, but it has 

 very similar characters (figure 1). The caudal lobe at its cephalic dorsal 

 angle also has a spur corresponding closely to the spur on the opposite side. 

 Small nuclear masses occur just above this spur. These are not found on 

 the left side. 



NUCLEUS OLIVARIS ACCESSORIUS DORSALIS. 



The nucleus olivaris accessorius dorsalis sinister is a thin sheet of cells, 

 about one-third the length of the olive and slightly less than one-half its 

 transverse diameter. As a more or less rectangular plate, it overlies dorsally 

 the second and third dorsal lobes, and is represented in outline in figures 5, 

 6, and 7. It is shown in part in figure 4. Beginning caudally just cephalad 

 to the third transverse fissure (r) of the dorsal surface it rapidly widens to 

 its line of greatest transverse diameter over the second dorsal fissure (6) 

 and thence continues cephalad in a slightly narrowing body to an abrupt 

 ending over the first dorsal fissure. Its mesial l)order at the inferior end 

 lies slightly lateral to the dorsal border of the hilum, Init soon approaches 

 and corresponds to that border. Hence, the dorsal accessory olive lies over 

 the dorsal middle and mesial third of the main nuclear mass. Dorsally, it 

 is covered by the formatio reticularis. The surface of the dorsal accessor}^ 

 olive is smooth, but it shows a slight convexity ventrally, the point of greatest 

 convexity being over the deep second dorsal transverse sulcus (6). The 

 right nucleus, as far as can be made out, corresponds fairly exactly to the 

 left. The nucleus has approximately the same extent as that in the new- 

 born, as reconstructed by JNIiss Sabin. 



THE PONTINE COMPLEX. 



In this reconstruction, the nuclear material of the caudal portion of the 

 pons, of the nuclei arciformes, and of the corpus ponto-bulbare was mod- 

 eled. The description of these structures will deal with these as portions 

 of a single morphological unit, as they are undoubtedly portions of the same 

 primary cell-mass. Essick (1912) has shown that the pontine miclei are 

 formed from cells migrating in the embryo from the Rautenlip|)e of His in 

 the lateral wall of the fourth ventricle, along the course of the corpus ponto- 

 bulbare, which he first described in 1907. This, then, necessarily includes 

 the corpus ponto-bulbare in the pontine nuclei. Somewhat similarly the 

 arcuate nuclei are formed from cells which migrate superficially around the 

 surface of the medulla from the Rautenlippe. These arcuate nuclei are of 



