The Lower Portion of the Human Brain-Stem. 21 



plate, lying superficially beneath the surface. Caudally it is wholly com- 

 posed of gracile nucleus (figure 10), but at the level of the transverse bridge 

 of cells, in the cuneate nucleus, the plate begins to be composed of the 

 nucleus fasciculi cuneati. At the caudal limit of the nucleus vestibularis 

 medialis (figure 3) the whole dorsal plate is occupied by cuneate cells. Then, 

 as the descending j^ortion of the vestibular nerve develops, the dorsal plate 

 recedes laterally in a lateral convexity to approach mesially and terminate 

 ventrally to the lateral portion of the nucleus vestibularis medialis. The 

 lateral margin of this dorsal plate is quite irregular (figui-e 3) and overhangs 

 (/. p., is dorsal to) the base of the lateral surface triangle. 



Above the superior transverse limit of the lateral triangle of the nucleus 

 fasciculi cuneati is the cephalic field of the lateral surface of the nucleus. 

 This is a fairly smooth area with a gradually convex upper border made 

 l)y the cells accompanying the descending vestibular root as they become 

 superficial. ^lesially the superior termination curves ventrally to the lateral 

 portion of the nucleus vestibularis medialis, to merge with the cephalic end of 

 the dorsal plate. 



The ventro-lateral limit of the nucleus fasciculi cuneati, as soon as the 

 nucleus reaches its average dimensions, is easily distinguished, on histological 

 grounds, from the adjoining substantia gelatinosa (figures 10 and 11); it 

 forms a fairly straight line following the dorso-mesial edge of the substantia 

 (figure 2). Mesially the nucleus fasciculi cuneati in its caudal half is well 

 separated from the nucleus of the fasciculus gracilis by dense fiber bundles. 

 In the cephalic portion of the gracile nucleus the differentiation of the two 

 nuclei is not as easy, but is possible. This line of separation is curved, 

 following the direction of the fibers coursing between the nuclear masses. 

 Mesially the nucleus fasciculi cuneati shows a curving smooth surface which 

 looks laterally and mesially and moves somewhat dorsally as it develops 

 cephalad. Its convexity follows the curving internal arcuate fibers. The 

 mesial ventral border, separating from the nucleus fasciculi gracilis, lies just 

 dorsal to the nucleus tractus solitarii (cf. figures 10 antl 11). Cephalad the 

 mesial border is abruptly pushed laterally by the developing pars descendens 

 of the vestilnilar complex; its mesial border, however, maintains its con- 

 vexity until it terminates superficially in the convex cephalic border of the 

 smooth superior lateral field. 



RELATION OF NUCLEI TO FLOOR OF FOURTH VENTRICLE. 



The correlation of the anatomical markings of the fioor of the fourth 

 ventricle with the underlying nuclear masses was first well worked out b}' 

 Streeter (1903). In this reconstruction, it has been attempted to show this 

 relationship in a slightly different manner than demonstrated by Streeter. 

 The work on this adult brain-stem was rendered less accurate }\\- the fact 

 that no drawings or photographs of the external form and of the floor of the 



