Surface view of mem- 

 branous semicircular 

 canal of cat : _r, fibrous 

 tissue supporting single 

 layer of polyhedral epi- 

 thelial cells (y). 



THE ORGAN OF HEARING. 



epithelium to constitute the perceptive apparatus of the semicir- 

 cular canals. 



These specialized areas are limited to the floor of the ampullae, 

 in which position the fibrous wall of the canal is 

 distinguished by a local thickening forming the 

 transverse ridge, or septum transversum. 



On approaching the base of the crest the epi- ' 

 thelial cells become more columnar, being much 

 taller and narrower than those of the general sur- 

 face. The specialized cells crowning the sum- 

 mits of the cristae acusticae, like other examples 

 of neuro-epithelium, consist of elements of two 

 kinds, the sustentacular or fibre cells and 

 the hair-cells. 



The sustentacular elements resemble those 

 of the maculae of the saccule and the utricle, 

 extending the entire thickness of the epithelial 

 layer and presenting an elongated narrow 

 irregular cylindrical body, with prominent projecting oval nucleus. 



The hair-cells, or auditory cells, reach only part-way to the 

 basement-membrane and bear on their free surfaces enormously long 

 hair-processes, the auditory hairs, which project at least as far 

 as the middle of the lumen of the ampulla. The auditory hairs 

 spring from the ends of the cells by minute conical expansions, and 

 under high amplification are resolvable into a number of finer com- 

 ponent hairs. The nuclei of the auditory cells usually lie within 

 dilated rounded inner extremities of the cells, with which also the 

 terminations of the auditory nerve come in close relations. 



In sections of the ampullae in tissue preserved with approved 

 reagents the inner free ends of the auditory hairs are embedded 

 within a peculiar dome-like structure, the cupola, regarding whose 

 nature, and even existence during life, opinions greatly differ. As 

 usually seen in well-preserved tissue, this structure appears as a 

 faintly-striated cuticular formation covering in the ends of the 

 hair-processes. 



The fibres of the auditory nerve pass into the septum trans- 

 versum, where they unite into net-works from which finer diverging 

 fibrillae pass into the overlying epithelium after losing their medul- 

 lary substance. Small groups of naked axis-cylinders extend be- 

 tween the epithelial cells and separate into the individual fibrillae, 

 some of which are applied to the bases of the hair-cells, while others 

 apparently seek their ultimate distribution at higher levels. 



The blood-vessels supplying the semicircular canals comprise 

 those destined for the bony capsule and those distributed to the 



