588 Mr. C. W. Andrews on a Cast of the 



cave on either side, the concavities indicating the projection 

 inwards of the auditory region of the skull. 



The ventral region of the medulla oblongata (med.) is 

 bounded anteriorly by the transverse groove above mentioned. 

 Longitudinally its ventral surface is at first convex and pos- 

 teriorly concave ; from side to side it is convex throughout 

 its length, tlie mid-ventral line being marked by a slight 

 ridge. Anteriorly the sides of the cast of the medullary 

 region pass up into the cerebellar prominence ; in its middle 

 portion its upper and lateral surfaces are impressed by the 

 prominent auditory region. Posteriorly it widens out con- 

 siderably and is roughly triangular in section, the angles of 

 the triangle being rounded and corresponding to the mid- 

 ventral ridge and the supero-lateral borders ; the upper surface 

 of this posterior region is convex. The cast no doubt gives 

 an accurate idea of the actual form of the medulla so far as 

 its ventral and lateral portions are concerned ; but it is pro- 

 bable that dorsally there was a considerable space between 

 the overhanging cerebellum and the upper surface of the 

 medulla, so tliat here the real shapi3of the latter is not shown, 

 no trace of the position of the fourth ventricle and other 

 structures being seen. Further back, however, where it is 

 about to pass into the spinal cord, it probably nearly or quite 

 filled the cranial cavity, and its true form is therefore shown 

 by the cast. 



The determination of the roots of the cr uiial nerves arising 

 from the medulla presents some difficulties, but by comparison 

 with the crocodilian brain, and by examining the relations of 

 the various foramina to one another and to the surrounding 

 structures, it has been possible to determine most of them 

 with a considerable degree of certainty. On the whole, these 

 agree with the determinations of Hulke, who, however, 

 employs the older terminology of Willis in his description. 



On the ventral surface of tlie medulla close to its anterior 

 end is a slight prominence, which probably marks the point 

 of origin of the abducens (VI.). Above and a little behind 

 this on the side wall of the medulla is the large root of the 

 trigeminal (V.). This passes out of the skull by a very large 

 foramen which opens externally into a fossa, from which 

 a deep channel runs forward along the inner wall of the 

 orbit, and, no doubt, lodged the ophthalmic branch (V.). 

 Behind the trigeminal foramen the skull-wall is perforated by 

 a small opening, which enlarges towards the outside and is 

 continued on the side of the cranium as a well-marked groove 

 {car.?) which runs downward and somewhat forward. Its 

 lower end passes round to the external opening by which the 



