298 THE SENSORIAL FUNCTIONS. 



There are, therefore, two qualities in sound recognisable 

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

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

 the latter, on their frequency. These acoustic principles 

 are to be borne in mind in studying the comparative physi- 

 ology of hearing; and since the functions of the different parts 

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

 stood, I shall, in treating of this subject, deviate from the 

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

 structure of the ear in its most perfectly developed state, 

 which 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 Concha, or external ear (c,) is formed of an elastic 

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



* In this and all the following figures, the parts of the right ear are shown, 

 and similar parts are always indicated by the same letters. 



