490 



ERNEST W. L. HOLT, 



t.v. i. 



The lobi inferiores (figs. 16 and 16 a li.) are well-marked, with 

 peripheral vesicular and internal fibrous matter. The oculomotor nerves 

 (J/J, fig. 16) and longitudinal fibrous bands of the medulla are un- 

 changed. The latter, in the anterior part of their course, present the 

 appearance of circular pale areas (fig. 16 a/". Z.) amongst the vesicular 

 tissue on either side of the aqueduct of Sylvius (aq). From the point 

 of origin of the auditory nerves, unchanged since the last stage 

 (cf. fig. 17), the ganglionic areas (a.g.) persist back on either side 

 of the fibrous bands to the hinder end of the medulla. 



Where the cerebellum passes into the roof of the fourth ventricle, 

 the brain case appears square in section. The narrow aqueduct of 

 Sylvius (fig. 16 a aq.) opens posteriorly into the fourth ventricle (as 

 in earlier and later stages , cf. woodcut - fig. 3 and fig. 18) , which 

 appears in transverse section as a deep dorsal fissure of the medulla 

 (woodcut-fig. 4 V. 4.), the lateral elements of which, forming the side 



walls of the ventricle, are raised up into con- 

 spicuous rounded mounds, the lobi posteriores 

 (l.p.)', the latter are connected by the thin 

 cellular roof of the ventricle {t.v.4.\ arising 

 from their dorso-lateral regions. As in the 

 next stage the lobi (cf. fig. 18 l.p.) do not 

 extend far back. The lateral elements of the 

 medulla (walls of the fourth ventricle) soon 

 approach each other dorsally, so as to shut 

 off the lower part of the brain cavity as the 

 central canal. The walls recede again a little 

 further back, leaving once more an undivided 

 cavity in transverse section (as in woodcut 4), 

 but the whole cavity gradually narrows, whilst 

 the thin cellular roof of the ventricle {tv.4.) 

 is gradually drawn in closer, and, opposite 

 the last trace of the auditory capsule, the walls are again dorsally 

 approximated, and the fourth ventricle finally disappears. The central 

 canal is continued back into the spinal cord. 



Behind the lobi posteriores, just after the first shutting off of the 

 central canal, a small nerve (Cr. IX) springs from the dorso-lateral 

 region of the medulla, and runs backwards between that structure and 

 the brain case. Towards the hinder end of the otocystic region, the 

 chondrocranium is pierced on either side of the parachordal portion, 

 by a single elongated foramen, which affords exit along a narrow and 



Fig. 4 . Vertical trans- 

 verse section of Medulla ob- 

 longata of I inch herring. Some- 

 what diagrammatic, a g. gan- 

 glionic area, f.l. longitudinal 

 fibrous bands, l.p. lobus poste- 

 rior, t.w. 4. roof of fourth ven- 

 tricle, V. 4. fourth ventricle. 



