364 CHARLES CLIFFORD MACKLIN 



of the cochlear duct, and, indeed, it is owing to this circumstance 

 that this part of the duct is thrust outward to form the pro- 

 montory on the outer surface. From a point just ventro-cranial 

 to the tip of this pyramid a small commissure, known as the 

 commissura laminopyramidalis (fig. 8), springs to join the lamina 

 spiralis which is immediately ventral, and this commissure 

 passes over the uncoiled part of the cochlear duct; at the same 

 time it divides the crescentic fissure in the floor of the internal 

 acoustic meatus into ventral and cranial parts. The caudo- 

 ventral surface of the pyramidal mass forms the medial wall of 

 the first part of the cochlear duct; the cranio-ventral surface 

 constitutes the medial and steepest part of the floor of the internal 

 acoustic meatus, while the border between these delimits later- 

 ally the slit-like foramen, piercing the ventralmost part of the 

 meatus, which transmiits the cochlear division of the acoustic 

 nerve. Dorsally the base of the pjo'amid is seen to be pierced 

 from above downward and backward by the foramen singulare 

 (fig. 9), which leads into the cavity of the ampulla of the poste- 

 rior semicircular canal, the region of exit appearing as an indenta- 

 tion of the inner wall just medial to the inner edge of the fenestra 

 perilymphatica. It appears in the cast of the cavity of the cap- 

 sule as a projection (fig. 9). A small portion of the dorsal side 

 of the pyramid is concerned in the formation of the ventral 

 wall of the vestibular space; the remainder, together with its 

 border joining the cranio-ventral surface, is directly continuous 

 with the cartilage of the medial wall of the vestibular part of 

 the pars cochlearis. 



The ductus cochlearis, shortly after passing the level of the 

 lamino-pyramidal commissure (fig. 8), emerges from the vestib- 

 ular part of the pars cochlearis, and enters the ventral, com- 

 pletely enclosed pars cochlearis (sensu stricto) which contains its 

 coiled part. The only entrances into the closed portion of the 

 cochlea are the passageways for the cochlear duct and the coch- 

 lear root of the acoustic nerve. The medial wall is here quite 

 thin, while the lateral wall presents the coiled lamina spiralis 

 (fig. 5). 



