AREAS OF THE MEDULLA OBLONGATA 875 



The formatio reticularis (Fig. 643) consists of diffusely scattered gray substance 

 in a meshwork of white fibres. It is far i/iore abundant in the medulla oblongata 

 than in the cord. In trans-sections of the medulla oblongata it is seen to be divided 

 by the hypoglossal nerve root fascicles into a mesal and a lateral field. In the 

 mesal field the gray substance is scanty, and white fibres principally longitudinal 

 ones preponderate; this is called the formatio reticularis alba in contradistinction 

 to the lateral grayer reticulated field, the formatio reticularis grisea. Its numerous 

 nerve cells mostly possess short axones and for the most part exercise associative 

 functions for the constantly active centres of respiration (nuclei of the vagus, 

 phrenic, facial, etc.). Certain axones of longer course are collected into a small 

 compact bundle just ventrad of the ventricular floor and central canal (and aque- 

 duct in the mid-brain), and known as the medial longitudinal fasciculus (posterior 

 longitudinal bundle). This tract is in intimate association with the cranial nerve 

 nuclei. The formatio alba is principally made up of this tract and the lemniscus 

 (interolivary stratum). 



The raphe' (Fig. 643) is situated in the middle line of the medulla oblongata 

 above the decussation of the pyramids. It consists of nerve fibres intermingled 

 with nerve cells. The fibres have different directions, which can only be seen in 

 suitable microscopic sections, thus: (1) Some run dorsoventrad; these are con- 

 tinuous with the external or superficial arciform fibres. (2) Some are longitu- 

 dinal; these are derived from the arciform fibres, which on entering the raphe* 

 change their direction and become longitudinal. (3) Some are oblique; these are 

 continuous with the internal or deep arciform fibres which pass from the raphe. 



The nerve cells of the raphe are multipolar; some are connected with the dorso- 

 ventral fibres, others with the superficial arcuate fibres. 



The restiform body succeeds the gracile and cuneate nuclei in the dorsolateral 

 part of the medulla oblongata. Its fibres converge from various sources and 

 ultimately enter the cerebellum as its inferior peduncle. For a description of these 

 fibre systems see the section on the "peduncles of the cerebellum." 



The nucleus of the olive or inferior olivary nucleus (nucleus olivarius inferior) 

 is a corrugated lamina of gray substance whose extent nearly corresponds to that 

 of the external elevation called the olive. It can be compared to a hollow oval 

 sac or purse, slit on its mesal aspect and the edges of the slit everted. The opening 

 is called the hilum. Numerous fibres stream into the interior through the hilum, 

 while others cut through the lamina to join the fibre arches of the reticular field 

 and then pass toward the restiform body. 



What are known as accessory olivary nuclei (nuclei olivarii accessorii) are smaller 

 detached or semidetached portions of the olivary nucleus named, according to 

 their position, the dorsal and medial accessory olivary nuclei (nuclei olivarii 

 accessorii, dorsalis et medialis). 



The olivary nuclei play an important part as relay stations in cerebellar con- 

 nections. A considerable mass of fibres, the olivocerebellar fibres (fibrae cerebello- 

 olivares), originate in the olivary nucleus of one side to enter the cerebellum 

 along the restiform body of the opposite side. A much less number of fibres, 

 running contrariwise, reach the olivary nuclei from the opposite cerebellar hemi- 

 spheres the cerebello-olivary (vestibulo-olivary tract} fibres. .Each olivary 

 nucleus is the terminus of the thalamo-olivary fibres, and Helweg's olivospinal 

 tract is believed to originate therein. 



The Arcuate Fibre Systems. The arcuate fibre systems comprise two sets of 

 fibres according as they course dorsad or ventrad of the olivary nuclei : 



1. The internal or deep arcuate fibres comprise the olivocerebellar fibres, just 

 described, and a number of commissural systems for the association of the teg- 

 mental reticular gray ganglia and cranial-nerve nuclei. Others pass cerebralward, 

 others to the cerebellum. 



