SKULL OF A HUMAN FETUS OF 40 MM. 361 



floor of the anterior subarcuate fossa, there is a small isolated 

 cavity, and as this is within the arch of the anterior semicircular 

 canal I regard it as a remnant of a portion of the otocyst, which 

 is, as yet, unclosed here. The ground substance of the cartilage 

 of this mass is abundant and pale staining, the nuclei being 

 relatively scattered, and surrounded by capsules not increased 

 in size. 



The remainder of the cartilage of the pars canalicularis is 

 made up of masses filling the interstices between the canals and 

 ampullae. 



The pars cochlearis is the anterior and smaller part of the otic 

 capsule, and lies immediately lateral to the upper end of the 

 basal plate (figs. 1 and 2). Like the pars canalicularis it is of 

 flattened, ovoid form, and contains the sacculus and ductus 

 cochlearis. Upon it we may recognize two principal surfaces, 

 medial and lateral, to which may be added a third or caudal 

 surface, made up of the structures in the ve^ibular portion 

 surrounding the foramen perilymphaticum. 



The medial and lateral surfaces are separated by a rounded 

 border, which runs from the perilymphatic foramen below, 

 around the ventral part of the pars cochlearis, over the cranial 

 pole, and thence backward to terminate by passing over the 

 suprafacial commissure to become continuous with the pars 

 canalicularis at the superior otic notch. The lowermost part 

 of this border is deflected outward to form the promontory; 

 within it is the first and uncoiled part of the cochlear duct, and 

 it is known as the prominentia cochlearis inferior (Voit) (figs. 

 2, 6 and 7). This prominence passes at first inward, forward 

 and upward, then almost directly upward to reach the cranial 

 pole, and finally passes backward into the prominentia coch- 

 learis superior (Voit) (figs. 1, 6 and 7) as the cranialmost border 

 of the pars cochlearis, which roofs the coiled part of the cochlea 

 (fig. 8), is called. Above the cranial pole the cartilago supra- 

 cochlearis appears (fig. 3). 



The lateral surface of the pars cochlearis (fig. 6) is smooth and 

 gently convex in its ventral portion, and here presents a shallow 

 groove, lying between the promontory and the cranial pole. 



