THE ACOUSTIC COMPLEX OF THE OPOSSUM 419 



nucleus may be said to "straddle" the spinal vestibular, so the 

 spinal vestibular in the region of origin of its two divisions may be 

 said to be squeezed between the two peduncle-like fiber tracts 

 associated with the cochlear reception nuclei. No sooner is the 

 pressure relieved than both the vestibular fiber-tracts expand con- 

 siderably, the anterior to its distribution in the cerebellar nuclei 

 and the superior vestibular nucleus, and the posterior to accom- 

 modate the cells of the spinal nucleus. 



Of the nuclear masses, the medial possesses perhaps the most 

 characteristic shape. It lies as previously remarked in the floor 

 of the fourth ventricle, its lateral border fused with the spinal 

 tract. The triangular area, apex posterior, which corresponds 

 to it, just lateral to the elevation marking the spinal tract is 

 easily recognized in the floor of the ventricle in the unsectioned 

 brain. The form of the dorsal surface and the longitudinal groove 

 in the ventral surface made by the sensory fifth, together with the 

 deep notch medial to it for the nucleus N. vagi and the tractus 

 solitarius are well shown in figs. 2 and 4. Anteriorly the medial 

 nucleus is pierced by the oiivo-cochlear tract in the manner 

 described above. The slender tongue extending horizontally 

 forward along the medial side of the olivo-cochlear tract toward 

 the region of the superior nucleus, will be noted again in connec- 

 tion with the sections. 



The nucleus of the spinal tract of the vestibular nerve takes 

 its form from that of the tract among whose fasciculi it lies. The 

 combined mass is therefore roughly a long cone, tapering towards 

 a blunt point posteriorly, but represented as cut across to allow 

 for as yet undetermined caudal extension. The cross section 

 posterior to the tuberculum acousticum is round or an oval slightly 

 flattened dorso-ventrally. In the region of greatest constriction, 

 the flattening of the dorso-medial and dorso-lateral surfaces and 

 the greater compactness of the mass is gained largely at the expense 

 of the grey matter, which is relatively small in amount here, 

 though most abundant just posterior to this region. The bound- 

 aries of the medial and spinal nuclei and the spinal tract are not 

 as distinct in the model as in the sections. Their form, however, 

 can be made out in figs. 5, 10 and 12. 



