580 A MANUAL OF VETERINARY PHYSIOLOGY 



Auditory Sensations. — Any analysis of these is hardly necessary 

 in a work dealing with the lower animals ; we have no direct 

 evidence that they understand or appreciate the difference 

 between music and noise ; a dog will howl at one as readily as 

 another. At the same time it is certain that animals can learn 

 to recognise sounds and associate them with certain ideas, as, 

 for instance, the commotion and excitement amongst the horses 

 of a regiment when the trumpet sounds ' feed,' and again the 

 recognition by a dog of its master's voice. Further, we have 

 undoubted evidence that sounds which are so feeble as not to 

 affect the human ear are readily perceived by some animals, so 

 that the acuteness of their sensations is greater than that of our 

 own, though their capacity for the enjoyment of music is absent 

 or extremely small. 



The vibrations set up in the tympanum are communicated to 

 the chain of bones, the stapes of which, through the fenestra 

 ovalis, imparts a push to the peri-lymph of the labyrinth ; this 

 fluid transmits the impulse through the vestibule, and from here 

 into the scala vestibuli of the cochlea. The vibrations ascend 

 the spiral staircase, and set in motion the membrane of Reissner, 

 which causes the lymph in the cochlear canal to vibrate ; when 

 these vibrations reach the summit of the cochlea they enter the 

 scala tympani through the helicotrema. The lymph in this 

 canal is now set in motion, with the result that the basilar 

 membrane, on which the organ of Corti rests, is affected, and the 

 vibrations are ultimately lost at the blind extremity of the 

 canal, whose membrane is pushed outwards at the fenestra 

 rotunda. Every push inwards at the fenestra ovalis causes, 

 therefore, a push outwards at the fenestra rotunda. During the 

 time the vibrations are crossing the cochlear canal from one scala 

 to another the organ of Corti is affected, and by means of the 

 auditory nerve the impulse is conveyed to the brain. It is in the 

 organ of Corti, with its nerve-endings, that the complex sounds 

 which make up even a single note of music are analysed, and 

 this analysis was at one time supposed to be effected by the rods 

 of the organ, which were believed to vibrate to their own par- 

 ticular tone, in the same way as a tuning-fork will pick out its 

 own tone from sounds in its vicinity and vibrate to it. This 

 view, tempting as it is, is negatived by the fact that the rods of 

 Corti do not exist in birds, and it has therefore been supposed 

 that the vibrations to the nerves terminating in the organ are 

 set up by the vibration of the basilar membrane on which the 

 organ is built, or by the tectorial membrane which covers them, 

 but the question is far from settled. 



