1076 HUMAN ANATOMY. 



the arcuate nucleus. The latter lies at first chiefly on the ventral and, higher, on the 

 mesial aspect of the pyramidal tract. The cells of this nucleus, small and fusiform, 

 are the origin of not a few of the superficial arcuate fibres, although those from the 

 dorsal nuclei continue their course over the nucleus without interruption. At the 

 upper end of the medulla, the cells of the arcuate nucleus increase in number and 

 mingle with those of the nucleus of the raphe and the pontile nucleus. 



Dorsal to the pyramid and immediately next the mid-line lies (4) the compact 

 tract of the median Jillct, composed of longitudinal fibres that are the upward continu- 

 ation of the deep arcuate fibres, which, from the sensory decussation to the upper 

 limit of the cuneate nucleus, bend sharply brainward after crossing the mid-line. The 

 fillet-tracts are also known as the interolivary stratum, as they constitute a compact 

 and laterally compressed field between the inferior olivary nuclei. Lateral to the 

 fillet, between the latter and the hypoglossal fibres, lies (5) the mesial accessory 

 olivary nucleus. (6) 'Y\\& posterior longitudinal fasciculus appears in cross-section 

 as a compact oval or laterally flattened strand, which lies next the raphe and 

 immediately beneath the gray matter covering the floor of the fourth ventricle. 

 This important path will be later described (page 11 16). The remaining space 

 of the anterior compartment, between the pyramid and the ventricular gray matter, 

 is occupied by the formatio reticidaris alba, so designated in distinction to the 

 formatio grisea on account of its meagre number of nerve-cells, since, with the excep- 

 tion of those scattered in the immediate vicinity of the mid-line ( nucleus raphe), few 

 cells are present. 



The Formatio Reticularis. — Repeated mention has been made of the reticu- 

 lar formation produced by the interweaving of the horizontal and vertical fibres. 

 Whilst particularly conspicuous within the medulla at the levels occupied by the 

 gracile, cuneate and inferior olivary nuclei, on account of the prominence of the 

 arcuate and cerebello-olivary fibres, the formatio reticularis does not end with the 

 disappearance of these nuclei and fibres, but is prolonged upward, although less 

 marked, by transversely coursing fibres derived from the reception-nuclei of various 

 cranial nerves — the vagus, glosso-pharyngeal, auditory, facial, and trigeminal — from 

 whose neurones axones of the second order arise that sweep across the mid-line 

 to join chiefly the fillet tract or to end, perhaps, about nerve-cells of other nuclei. 

 In this manner the formatio reticularis finds representation within the dorsal or 

 tegmental areas of the pons and the cerebral crura. The longitudinal fibres within 

 the formatio reticularis grisea are derived from many sources. Some are the 

 continuation of Gowers' tract ; some belong to the long strands concerned in 

 establishing refle.x paths connecting the corpora quadrigemina, nucleus rubrum, 

 vestibular and olivary nuclei with the spinal cord ; some are the axones of tegmental 

 neurones and pursue shorter courses, both descending and ascending, as association 

 fibres linking together different levels of the brain-stem ; while still others are the 

 prolongations of the spino-thalamic and other long tracts of the antero-lateral ground- 

 bundle of the cord. The longitudinal fibres of the formatio alba are chiefly the 

 components of the mesial fillet and of the posterior longitudinal fasciculus with, 

 possibly, the addition of short association fibres proceeding from the nerve-cells that 

 are found within the anterior area. 



The details of a transverse section passing just beneath the lower border of the pons (Fig. 

 932) vary considerably from those of the level shown in Fig. 930. The ventral half of the 

 medulla has lost in width in consequence of the disappearance of the superficial olivary emi- 

 nence, the inferior olive being at this level represented by only a few irregular plications. The 

 pyramids, likewise, are narrower, and separated by the broadened anterior median fissure. The 

 mesial fillet and the posterior longitudinal fasciculus are now widely separated by the inter- 

 vening nucleus centralis inferior that appears between them along the raphe. The nuclei of the 

 hypoglossal and glosso-pharyngeal nerves are no longer seen, but instead, along the floor of the 

 ventricle underlying the area acustica, appears a large triangular rrjass of gray matter, the 

 mesial vestibular fiucleiis. External to the latter the lateral or Dciters' nucleus and the 

 descending or spinal vestibular root lie close to the restiform body, which in transverse section 

 presents a bean-shaped outline. Between the restiform body and the descending trigeminal root, 

 the fibres of the mesial or vestibular part o\ the auditory nerve pass backward to gain the vestib- 

 ular nuclei. The outer surface of the restiform body is closely related to a considerable 



