THE MEDULLA OBLONGATA. 



287 



The descending root of the vestibular nerve is seen as a field of loosely grouped 

 bundles of cross-sectioned nerve-fibres to the inner side of the restiform body. 

 (3) The fasciculus solitarius shows as a conspicuous transversely cut bundle 

 that lies dorso-mesially to the vestibular root. (4) The descending root of 

 the trigeminal nerve is readily identified as a superficial crescentic field 

 enclosing on its mesial aspect the substantia gelatinosa. 



The lateral area, between the diverging vagal and hypoglossal root- 

 fibres, is occupied, in addition to (i) the inferior olivary and (2) the dorsal 

 accessory nucleus, chiefly by the feltwork of fibres producing the formatio 

 reticularis. In contrast to that within the ventral area, the reticulum of the 

 lateral area contains considerable diffuse gray matter between its fibres and, 

 hence, is known as (3) the formatio reticularis grisea. Accessions to the 

 irregularly distributed nerve-cells, for the most part large- and stellate or 

 fusiform, occur as two more definite collections; one of these, (4) the nucleus 



Nerve-eel 



Longitudinal 

 fibres 



FIG. 334. Portion of formatio reticularis grisea of medulla. X 130. 



ambigiws, consists of an inconspicuous group of large cells lying about the 

 middle of the gray reticular substance; it is important as the ventral motor 

 nucleus giving origin to at least a part of the motor fibres of the ninth and tenth 

 nerves. The other collection, (5) the micleus lateralis, includes an uncertain 

 aggregation of medium-sized cells situated near the periphery, ventral to the 

 trigeminal root. A separate group of somewhat larger neurones, near the 

 ventral border of the trigeminus, is the nucleus lateralis dorsalis. In a 

 general way, these nuclei (ambiguus and lateralis) of the substantia grisea 

 may be regarded as homologues of the lateral horn cells of the cord, just as 

 those of the hypoglossal nucleus resemble the anterior root-cells of the 

 spinal nerves. 



The ventral area, between the mid-line and the hypoglossal root-fibres, 

 is occupied ventrally by (i) the pyramidal tract, which appropriates the 

 entire width of the field with the exception of a very narrow peripheral zone 

 that intervenes between the pyramid and the surface along the median fissure 

 and the ventral aspect of the medulla. This zone is traversed by (2) the 

 anterior superficial arcuate fibres, among which lies an irregular column of 

 nerve-cells, (3) the arcuate nucleus. Dorsal to the pyramidal tract and next 

 the mid-line, lies (4) the compact tract of the median fillet, composed of 

 longitudinal strands that are the upward continuations of the deep arcuate 

 fibres that at lower levels have bent brainwards, after crossing the mid-line. 



