LEMUR MONGOZ 47 



as the descending tract of the fifth nerve (Trd), stand out as prominent 

 elements and add to the preeminent sensory character of the dorsal field. 

 Dorsal to the pyramidal tract and ventral to the fillet is an important 

 bundle of fillers, namely the posterior longitudinal fasciculus, which is 

 concerned with the automatic rellcxes for the regulation of certain primitive 

 movements. Lateral to the fasciculus longitudinalis posterior is a considerable 

 bundle representing the Dciterso-spinal tracts (DT), whose function is the 

 conduction of those impulses necessary to the adjustments ol equilibrium. 



LEVEL THROUGH THE NHDDLE OF THE LNFERIOR OLI\ E 

 (FIGS. 24 AND 25) 



At the level through the middle of the inferior olive, the outstand- 

 ing feature is the appearance of an irregular mass of gray matter lying 

 in the ventral iield near the pyramid. This is the inferior olivary nucleus 

 (10). Its significance has already been touched upon in relation to the. 

 automatic regulation of simultaneous movements of the head, eyes and 

 hand and to the facilitation of the coordination of all skilled learned perform- 

 ances. This structure has a direct connection, through the central tegmen- 

 tal tract, with the oculomotor nuclei and, through the olivary fasciculus of 

 Helweg, with the cervical levels of the spinal cord. By means of many arcuate 

 fibers it is also connected with the cerebellum. \\ hilc there is no diOiculty in 

 identifying this structure in lemur, it is smaller and much more indefinite in 

 outline than in most of the other species considered. The lemur thus appears 

 to be provided with but a small amount of automatic regulation of simultane- 

 ous movements of hands, head and eyes. Mesial to the olive and adjacent 

 to the midline of the section is the mesial fillet (Mf). This structure 

 represents the continuation of the discriminative sensory pathway, being 

 made up largely of fibers for sensation in the extremities. The substantia 

 gelatinosa (.NR) and the accompanying descending trigeminal tract 



