SENSE OF HEARING. 415 



ized nations, it is often weighed down, scarcely excite any sen- 

 sibility in it. It is essentially composed of a cartilage, folded in 

 a peculiar manner, which appears to constitute it an organ for 

 collection ; both its direction and its form are changed in ani- 

 mals by the action of intrinsic and extrinsic muscles, which 

 are brought into play according to the degree of attention 

 with which the animal listens to different sounds. In man, 

 these muscles are rudimentary, or, at the most, only the ex- 

 trinsic muscles contract with the fronto-occipital system, when 

 the attention to any sound is carried to its highest degree. 



This pinna is thus of little service for intensifying sounds, 

 and those who are deprived of it do not experience any 

 sensible change in their power of hearing. It appears, how- 

 ever, to be of use in enabling us to judge of the direction of 

 sounds: a person deprived of it, or who, for the sake of 

 experiment, renders it useless by flattening it forcibly against 

 the head, or by filling its convolutions with wax, will find 

 himself unable to distinguish the direction from which sounds 

 proceed ; there is no doubt that we judge of the direction 

 and origin of sounds by the slight modifications in their in- 

 tensity produced by the manner in which the sound-waves 

 strike the pinna, and are reflected from it. We judge of 

 their direction, also, by the fact that the sound does not 

 strike both ears alike : it rarely happens that we can distin- 

 guish whether a sound proceeds from directly before us, or 

 directly behind us ; we therefore turn the head slightly, and 

 incline the ear in the direction from which we suppose the 

 sound to come. 



The external auditory canal (meatus auditorius externus) 

 is of greater importance ; if this is obstructed, the sense of 

 hearing is diminished, and, if it is too much contracted, deaf- 

 ness sometimes follows. 1 It furnishes two methods by which 

 sounds may be transmitted: these consist of the column of 

 air inside it, and the cartilaginous and bony walls of which 

 it is formed ; these walls transmit the waves by vibration, 

 directly to the bones of the head, and thence to the fluid of 

 the labyrinth, and we can see how much easier the transmis- 

 sion must take place, since the vibrations spread through 

 solid mediums. This auditory canal is also remarkable for 

 its peculiar sensibility : at its opening are found large hairs, 

 and if these are touched, or any excitation brought to bear 



1 See P. Bonnafont, " Traite des Maladies de 1'Oreille." 1873, 

 p. 120. 



