THE ACOUSTIC COMPLEX OF THE OPOSSUM 415 



ful, at least in the opossum. The medial nucleus lies in the floor 

 of the fourth ventricle adjacent to the median line, and in close 

 association posteriorly with the nucleus N. hypoglossi, and below 

 with the nuclei of the N. vagus and the N. accessorius. Anteriorly 

 its relation to the cochlear nuclei varies with the position of these 

 in different brains. The cells of the nucleus of the spinal root as 

 the name indicates, lie among the fibers of the spinal root itself. 

 This brings its position immediately lateral to. the medial nucleus, 

 the adjoining borders of the two not being sharply differentiated 

 from each other. The lateral nucleus (Deiter's) apparently 

 consists of two parts as described by Sabin for the- human brain, 

 one medial and the other lateral to the spinal root of the vestibular 

 nerve, and both approximately opposite, the point of entrance of 

 the nerve. The division is only apparent however, since typical 

 Deiter's cells can be recognized among the strands of the spinal 

 tract, uniting the two divisions of the nucleus. The superior 

 nucleus of the vestibular nerve hes in the line of continuation 

 anteriorly of the vestibular spinal and ascending roots, in close 

 association with Deiter's, and so wedged between it and the 

 nucleo-cerebellar tract in higher mammals that it was at first 

 regarded as an appendage of that nucleus. Whatever may be 

 said for its definiteness in brains like the human, in the opossum 

 it is outlined with difficulty. The ascending tract is distributed 

 in part to the superior nucleus, as already mentioned, and in part 

 to the basal nuclei of the cerebellum, especially the nucleus fastigii. 



Decussation between the vestibular complexes of the two sides 

 of the brain occurs between the fibers of the nucleo-cerebellar 

 tract by way of the decussation of the two fastigial nuclei, and 

 anteriorly between fibers from the two superior nuclei, opposite 

 the point of origin of the brachium conjunctivum. 



The nucleus cerebello-acousticus is the name under which Cajal 

 has called attention to the existence of a group of cells in the lateral 

 wall of the ventricle, in the margin of the ascending limb of the 

 spinal tract as its cerebellar fibers ascend towards the ventricular 

 roof, which receive collaterals from the ascending root. Such a 

 structure cannot be definitely determined in Weigert preparations 

 of the opossum hindbrain. 



