PAPIO CVNOCEPHALUS 315 



ventral to the fillet. Ventrolateral to the griseal portion of the retieular 

 formation is the intermediate medullary substanee whieh eontains the 

 Deiterso-spinal tracts (DT) as well as the rubrospinal and spinothalamic 

 tracts (Rst, Spt). On the periphery of this region is a narrow band of 

 fibers, the circumferential zone, containing the ventral spinocerebellar tract 

 (Gow). Immediately contiguous to the ventrolateral surface of the inferior 

 olivary nucleus is the well-defined bundle of the central tegmental tract 

 (Ctt) which connects this nuclear mass with structures in the midbrain. 



LEVEL OF THE VESTIBULAR NUCLEI (FIG. I 48) 



Here a prominent alteration takes place. This change involves the 

 final disappearance of certain sensory elements which at lower levels are pre- 

 dominant features in the oblongata, with the substitution therefore of certain 

 sensory elements having a totally different function. The latter come 

 to occupy positions topographically identical with the sensory elements 

 which they now replace. If the dorsal portion of the oblongata of the lower 

 levels is notable for a single feature, it is the presence of the sensory relay 

 stations which take up and pass on those stimuli for discriminative sensibility 

 arising in the tail, in the extremities, in the trunk and neck. Having fulfilled 

 their mission by thus providing for the complete relay of these sen- 

 sory impulses, the nuclei of Goll and Burdach disappear in order to make way 

 for other sensory receiving stations which transmit impulses from proprio- 

 ceptive receptors of a highly specialized variety. These latter receptors are 

 lodged in the semicircular canals, in the utricle and saccule of the internal ear. 

 They are related to a function intimately concerned with the maintenance of 

 body balance. Two large nuclei, collectively forming the vestibular area 

 of the oblongata, thus replace the nuclei of Goll and Burdach. 



The second nuclear mass, by the addition of which this level becomes 

 notable, is situated in a still more lateral position and receives fibers also from 



