298 THE SENSORIAL FUNCTIONS. 



Tlicre are, therefore, two qualities in sound recognisable 

 by the ear, namely, loudness, or intensity, and quality, or 

 tone; the former depending on tlie force of the vibrations; 

 the latter, on tlieir frequency. These acoustic principles 

 arc to l)e borne in mind in studying the comparative physi- 

 ology of hearing; and since the functionsof the different parts 

 of the organ of this sense are, as yet, but imperfectly under- 

 stood, 1 sliall, in treating of this subject, deviate from the 

 plan I liave hitherto followed, and premise an account of the 

 structure of the ear in its most perfectly developed state, 

 wluch it appears to be in Man. 



§ 2. Physiology of Hearing in Man. 



That part of the organ of hearing, which, above all others 

 is essential to the performance of this function, is the acous- 

 tic nerve, of which the fibres are expanded, and spread over 

 the surface of a fine membrane, placed in a situation adapt- 

 ed to receive the full impression of the sonorous undulations, 

 which are conveyed to them. This membrane, then, with 

 its nervous filaments, may be regarded as the immediate or- 

 gan of the sense; all the other parts being merely accessory 

 apparatus, designed to collect and to condense the vibrations 

 of the surrounding medium, and to direct their concentrated 

 action on the auditory membrane. 



I have endeavoured, in Fig. 390, to exhibit, in one view^, 

 the principal parts of this complicated organ, as they exist 

 in man, in their relative situations, and of their natural size: 

 thereby affording a scale by which the real dimensions of 

 those portions, which I shall afterwards have occasion to 

 explain by magnified representations, may be properly ap- 

 preciated.* 



The Coiicha, or external ear (c,) is formed of an elastic 

 plate of cartilage, covered by integument, and presenting va- 



• In lliis and uU the following' figures, the parts of the right car are shown, 

 and similar parts arc always indicated by the same letters. 



