The Lower I'ortioH of the Hiotnin Bntln-Slcni. 29 



irregularities, the nucleus presents the following surfaces, more or less limited 

 in extent: mesial, dorso-mesial, dorso-lateral, lateral, and a ventral border 

 which widens into a ventral surface. 



From mesial view (figure 4) the chief characteristics of the nucleus can 

 be made out, as this view shows the dorsal convexity of the nucleus. Like 

 the nucleus nervi hyjioglossi, the nucleus ala* cinereje shows a dorsal angle, 

 occurring about the middle of its dorsal border. This underlies the eminence 

 in the ala cinerea of the floor of the fourth ventricle and is quite marked, as 

 shown in the drawing. Traced caudally from this point, the dorsal border 

 runs ventrally, caudally, and mesially to reach a jjosition close to the mid- 

 line at the inferior extremity of the fourth ventricle. From this point 

 caudally, the border runs caudally and ventrally, but not as markedly ven- 

 trall}^ as just above. This change in direction of the dorsal border of the 

 nucleus results in the formation of an oblique angle with the apex dorsally. 

 Above the dorsal angle of the nucleus, the dorsal border is continued cephalad 

 and slightly ventrally and laterally, this direction taking the nucleus ventrally 

 to the dorsally projecting nucleus vestibularis medialis. This cephalic half 

 of the dorsal border shows three gentle ventral depressions, the most cephalic 

 being most marked and terminating in the cephalic dorsal bulging of the 

 superior pole. 



The dorso-mesial surface is that part of the nucleus which presents 

 superficially beneath the floor of the fourth ventricle. It shows as a slightly 

 convex face on mesial view (figure 4), which in general looks mesially and 

 somewhat dorsall.y. It is marked off from the mesial surface by a change 

 in the planes of the surfaces which causes a slight angle to be formed in the 

 shape of a continuous line running from the ventricular limit of this face 

 downward and ventrally in a gradual dorsal convexity. This forms the base 

 of a triangle w^hich the dorso-mesial surface composes. The line between the 

 dorso-lateral and mesial surfaces at the inferior limit of the ventricular 

 portion of the nucleus splits into two lines a direct caudal line, short and 

 marking the ventricular limit, and a prolonged, indefinite, ventrally curving 

 line which goes to the caudal limit of the nucleus. Between the two lines is 

 a small, smooth, irregularly' triangular surface which looks directly mesially. 



Between the dorsally convex line separating the dorso-mesial surfaces 

 and the ventral border is the irregularly shaped mesial surface (figure 4). 

 In its cephalic portion this surface looks almost exactly mesially, but a 

 gradual rotation occurs, so that in the lower jiortions the surface looks 

 obliciuely ventro-mesially (figure 10). The upper extra- ventricular portion 

 of the surface is narrow and somewhat triangular, the apex of the triangle 

 being contained in the cephalic enlargement or knob. Below this narrow 

 upper portion the nucleus forms a fairly accurate elongated rectangle, nar- 

 rowing somewhat, however, in the caudal portion. The surface is marked 

 by several rather superficial grooves and by one gradual but somewhat 

 extensive dorso-lateral depression just caudal to the middle of the nucleus. 



