366 CHARLES CLIFFORD MACKLIN 



and passes dorsally into the medial edge of the suprafacial 

 commissure. 



The floor of the meatus is composed of three distinct portions; 

 the ventral, formed by the first part of the lamina spiralis, the 

 dorsal, which is the ventral edge of the wall of the vestibular 

 portion in this region, and the medial, formed by the cranio- 

 ventral surface of the pyramid, as we have seen. These surfaces 

 increase in steepness in the order mentioned, so that the cres- 

 centic fissure (fig. 7) formed by their approximated deep edges, 

 is deeper caudo-ventrally than cranio-dorsally. Looked at from 

 within the ventral and dorsal are the only surfaces visible, the 

 ventral presenting much the greater area. The borders of the 

 latter where they join with the margins of the meatus present 

 the enlarged extremities of the crescentic fissure, the upper of 

 which serves for the passage of the facial nerve, the lower and 

 anterior for the upper part of the cochlear division of the acoustic 

 nerve. 



Five foramina appear in the internal acoustic meatus, and of 

 these the foramen singulare has been considered. It transmits 

 the posterior ampullary nerve to the inferior cribriform macula. 

 The other four are parts of the crescentic fissure. This latter is 

 divided into almost equal limbs by the lamino-pyramidal com- 

 missure, which has been noted overlying the first part of the coch- 

 lea, and joining the dorsal surface of the first part of the spiral 

 lamina with the pyramid. The ventral limb is long and slit-like, 

 widest in its ventro-cranial end, and transmits the fibres of the 

 cochlear root of the acoustic nerve. It will later become the 

 spiral foraminous tract. The cranial limb is separated by two 

 cartilaginous septa into three foramina, the upper, which we 

 have seen, being large, and transmitting the 7th cranial nerve, 

 and being known as the facial foramen, the lower two being of 

 about the same size, and transmitting the superior and inferior 

 branches of the vestibular root of the acoustic nerve (figs. 5 

 and 7). 



I have also reconstructed a model of the cavity of the otic 

 capsule, and from the illustrations of this a conception of the 

 general plan of the cavity may be gained (figs. 8 and 9). This 



