THE MEDULLA OBLONGATA. 



285 



with this decussation, for, on the contrary, it is only the beginning- of an 

 extended series of afferent arcuate fibres that pass across the mid-line at 

 various levels throughout the brain-stem. Since many longitudinal fibres 

 are encountered by those sweeping from side to side, an interweaving of 

 vertical and horizontal fibres takes place, which results in the characteristic 

 formatio reticularis that constitutes a large area within the medulla (Fig. 

 333), as well as within the dorsal or tegmental portions of the pons and 

 cerebral peduncles. 



The olivary nuclei include, in each half of the medulla, three masses 

 of gray matter the inferior olivary nucleus and the two accessory olivary 

 nuclei. The inferior olivary nucleus is a corrugated sac-like lamina of gray 

 matter which underlies and causes the conspicuous oval elevation, the oliva, 



Cerebello-olivary strands 



Gray 

 matter 



Core of 



white 



matter 



FIG. 332. Section of inferior olivary nucleus, showing plicated sheet of gray matter traversed by strands 



of nerve-fibres. X 100. 



that occupies the upper half of the medulla at the outer side of the pyramid. 

 In favorable cross-sections, the nucleus appears as a striking sinuous C-like 

 figure (Fig. 333), the mouth of the sac or hilum looking mesially. The 

 greatest length of the nucleus is from 12-15 mm. and the transverse diameter 

 about half as much. The plicated lamina of gray matter, from .2-. 3 mm. 

 thick, contains numerous small rounded neurones, from 1826;;. in diameter, 

 each provided with from three to five branched dendrites and an axonCj 

 embedded within a complex of axis-cylinders and neuroglia. The interior 

 of the gray sac is filled with white matter, consisting of nerve-fibres, that, 

 for the most part, stream through the hilum and constitute the olivary 

 peduncle. The accessory olivary nuclei are two irregular plates of gray 

 matter that lie median and dorsal to the chief olive. The median nucleus, 

 iq-i i mm. long, is sagittally placed between the tract of the fillet and the 

 root-fibres of the hypoglossal nerve. The dorsal nucleus is less extensive 

 than the median and lies close to and behind the hilum of the inferior olive. 

 In structure the accessory nuclei resemble the gray matter of the chief one. 



