FUNCTION OF THE COCHLEA. 373 



sound, one having been communicated by the fenestra ovalis, the other 

 through the fenestra rotunda, their common point of convergence, and 

 perhaps of mutual destruction, being at the helicotrema, the aperture at 

 the apex through which they intercommunicate. Nor can we fail to be 

 struck by the circumstance, if this explanation of the function of the 

 scalse be correct, in what an admirable manner the whole in- . d . tment of 

 strument is provided with self-adjusting power, since, when the length of 

 the stirrup forces in the membrane of the fenestra ovalis, the t] 

 pressure which is communicated through the water pushes out the mem- 

 brane of the fenestra rotunda, and thereby the relative length of the two 

 scalas has changed, the one having become longer by as much as the 

 other has become shorter, an adjustment necessary to bring about total 

 interference at the helicotrema. And we might add that. such a con- 

 struction is all the more interesting, for, since it is the intensity of the 

 waves that is to be destroyed, reliance is had upon the intensity instru- 

 ment, the drum, to produce that effect, and it is done by the contractions 

 of the tensor tympani and stapedius muscles. Perhaps the perfect ac- 

 complishment of this interference is the standard, to which allusion has 

 been made before, by which the mind judges of the power which has been 

 put forth by those muscles, and thereby of the intensity of the sound. 



From the comparative anatomy of the cochlea and the character of the 

 vocal organs, JVI. Duges formerly came to the conclusion that Com arative 

 the cochlea has for its function the determining of the pitch anatomy of the 

 of sounds. In man, whose vocal powers are most varied, it Cl 

 exists in the highest perfection ; in birds, whose vocal powers are more 

 restricted, it is reduced to a short and slightly curved tube, but still di- 

 vided by a longitudinal septum ; in reptiles, it exists only in a rudiment- 

 ary state. 



The necessary existence in the ear of some mechanism for the purpose 

 of preventing reverberation or repercussion has long been recognized 

 both by writers on acoustics and by physiologists. Thus an explana- 

 tion of the functions of the semicircular canals and of the cochlea upon 

 this principle is given by Dr. Koget, in his Bridgewater Treatise ; and, 

 in like manner, Professor Jackson, of Philadelphia, has for several years 

 taught a similar doctrine in his lectures. Since 1840, 1 have in my pub- 

 lic lectures presented the explanation given in the preceding paragraphs. 

 It differs essentially from that of my friend, Professor Jackson (of which 

 a brief statement may be found in Dr. Smith's edition of Carpenter's 

 Physiology, Philadelphia, 1855), in this, that it limits the accomplish- 

 ment of interference to the cochlea. The view which I entertain respect- 

 ing the function of the semicircular canals will be immediately set forth : 

 it does not appear to me that they are in any way connected with the in- 

 terference mechanism. 



