892 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



The vibrations communicated to these structures in some way give 

 rise to nervous impulses passing into the terminal filaments of the 

 auditory nerve. As to the way in which this is accomplished but little 

 is known. The temptation is strong to find the receiving apparatus 

 in the organ of Corti, which is composed of a long series of rods 

 varying regularly in length and in the span of their arches. The 

 analogy between these structures and, for example, the strings of the 

 piano is very striking. As is well known, a musical tone sounded 

 in front of an open piano will set into vibration the corresponding 

 string of this instrument. The temptation is almost irresistible to 

 suppose that a similar mechanism is concerned in the perception of 

 different sounds. If we could imagine that certain definite parts 

 of the organ of Corti were thrown into vibration only by appropri- 

 ate sounds the complex process of the perception of different musical 

 intervals would be greatly simplified, but the more the subject is 

 examined into the greater are the difficulties surrounding such an 

 explanation. 



In the first place, the terminal filaments of the auditor y nerve have 

 been traced to the inner and outer hair-cells, and it must, therefore, be in 

 this locality and not in the rods of Corti that the sensory impulses 

 commence. 



In the second place, the rods of Corti are entirely absent in birds, 

 who, without doubt, are capable of appreciating musical sounds; while, 

 again, the variation in length of the rods of Corti would be insufficient to 

 explain the great scope which the ear possesses in the recognition of the 

 pitch of sounds. 



On the other hand, the basilar membrane- is tense in a radial 

 direction and loose longitudinally, and, therefore, as Helmholtz has 

 suggested, may be compared to a series of strings of varying tension 

 and length. 



If this basilar membrane be looked upon as the receptive organ we 

 must then assume that each vibration travels up the scala tympani, 

 throws into sympathetic vibration a small part of the basilar membrane, 

 which transfers the vibration to the sensory structures above it. 



In support of this view it may be mentioned that the radial dimen- 

 sions of the basilar membrane offer a wider field of difference than 

 we find in the length of Corti's rods. The whole subject is, however, in 

 the highest degree obscure, and all that we can say is that the organ of 

 Corti, composed of the basilar membrane, the rods, and hair-cells, is in 

 some way concerned in the reception of sound-waves ; the manner is, 

 however, entirely unknown. After all, it must not be forgotten that the 

 perception of sound takes place not in the ear but in the brain, and that 

 sound-waves, received in whatever way by the terminal filaments of the 



