Hearing, and the Ear. 459 



panic membrane, sounds have been communicated to the fenestra ovalis 

 by way of the throat and this Eustachian tube. The inside of the two 

 sacs is lined with epithelium, or the sensitive continuation of the out- 

 side epidermis or skin, which, as shown elsewhere, is, during the process 

 of embryonic development, pushed in, or invaginated from the outside, 

 and afterwards pinched off or detached from the epidermis as sacs. In 

 the inside of the ampullae, there is a fold or ridge of the epithelium, 

 which is called the Crista Acustica, or acoustic crest. The fibres of the 



FIG. 195. Longitudinal Section of a Human Ampulla through the Crista Acustica* 

 Semi-diagrammatic. 



p .Cavity of the Ampulla filled with Endolymph. 

 v. Crista Acusticathe letter marks the lower limit of the auditory epithelium. 

 au Columnar cells of the auditory skin or epithelium. 

 h . Auditoiy hairs projecting into the Cupula. 

 cup term The Cupula Terminalis. 

 n. Nerve fibres entering the base of the Crista and passing into the columnar or auditory 



The Cupula Terminalis in which the hairs terminate is a soft material of sera i-fibri liar 

 structure. Thus embedded, the ends of the hairs do not vibrate singly, but the agi- 

 tation of the endolymph must vibrate all. (From Quain After E. A. Schafer.) 



auditory nerve penetrate this ridge from the outside, and communicate 

 through the cells which cover the ridge with a great number of stiff 

 bristles which stand on the ridge. These bristles extend outward into 

 the endolymph which fills the ampullae as well as the connecting canals, 

 and whenever the endolymph is vibrated the motion is communicated to 

 the hairs and by them transferred to the cells. From the cells it passes, 

 as a nerve current, up the fibres of the auditory nerve to the brain. 

 (See fig. 195.) There are some of these auditory hairs also in the body 

 of the membranous sac with which the canals connect. In this part of 

 the sac there are also a number of calcareous crystals called otoliths 

 ear-stones. These are suspended by nerve filaments and intensify the 

 agitation of the nerves when the fluid is vibrated. The auditory nerve 

 divides into two principal branches, one of which is called the vesti- 



