THE NERVOUS SYSTEM 1377 



Ventral or Anterior Wall. — ^This wall is usually referred to as the 

 floor. It is formed by (i) the dorsal surface of the upper or open 

 half of the bulb, and (2) the dorsal surface of the pons Varohi, 

 between the converging superior peduncles of the cerebellum. In 

 shape it is rhomboidal, the bulbar and pontine parts being triangular 

 and having their bases applied to each other. The floor is some- 

 times spoken of as the fossa rhomboid ea. 



The lower etid is tapering and lies between the clavae of the 

 funiculi graciles. It constitutes the calamus scriptorius. In this 

 situation the ca\'ity of the ventricle is continuous with the central 

 canal of the spinal cord, after that canal has traversed the lower 

 or closed half of the bulb. The upfer end is somewhat tapering 

 and lies between the convei^ng superior peduncles of the cere- 

 bellum. In this situation the ca\'ity of the ventricle is continued 

 into the aqueduct of Syhaus, which traverses the mesencephalon 

 and opens superiorly into the third ventricle. 



The floor is widest across its centre, which is on a level with the 

 upper ends of the restiform bodies of the bulb. The cavity of the 

 ventricle is here prolonged on either side roimd the outer aspect 

 of the corresponding restiform body towards the olive. This pro- 

 longation is known as the lateral recess. 



The floor is covered by a thick layer of grey matter, which is 

 continuous with the central grey matter of the lower or closed half 

 of the bulb. This grey matter is covered b}' ependyma, the epi- 

 thelial cells being continuous with those which line the central 

 canal of the spinal cord. 



The floor is traversed in the median line by a slight longitudinal 

 groove, which divides it into two s\Tnmetrical longitudinal halves. 

 Each half is crossed at its widest part by bundles of white fibres, 

 called sir ice acusticcB. They wind round the upper part of the 

 restiform body, and pass transverseh' across the corresponding 

 half of the floor as far as the median longitudinal groove, into 

 which they sink. They belong to the cochlear division of the 

 auditory nerv-e, in connection with which they will be described. 



The floor is di\'ided into two parts, lower and upper, by the 

 striae acusticse of either side. 



Lower or Bulbar Part. — The bulbar part is formed by the dorsal 

 surface of the upper or open part of liie bulb. It is bounded on 

 either side by (i) the clava of the funiculus gracilis, (2) the cuneate 

 tubercle of the funiculus cuneatus, and (3) the restiform bod\% in 

 this order from below upwards. The lower tapering end consti- 

 tutes, as stated, the calamus scriptorius. The bulbar part is trav- 

 ersed in the median line by a longitudinal groove, already referred 

 to, and this groove subdivides it into two symmetrical halves. 



Inunediately below the striae acusticae on either side, and not 

 far from the median longitudinal groove, there is a small triangular 

 depression, known as the fovea inferior. Its apex extends to the 

 striae acusticae, and the lateral angles of its base are prolonged 

 dowTiwards as two grooves — dinner and outer. The inner groove 

 passes in a somewhat curved manner towards the point of the 



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