764 SPECIAL SENSES. 



sounds conveyed to the cochlea throw into vibration only those elements of 

 the organ of Corti which are tuned, so to speak, in unison with them. Ac- 

 cording to this hypothesis, the rods of Corti constitute a harp of several 

 thousand strings, played upon, as it -were, by the sonorous vibrations. The- 

 ories analogous to the one proposed by Helmholtz, but of course lacking the 

 basis of exact anatomical and physical details developed by modern researches 

 and experiments, were advanced by Du Verney (1683) and by Le Cat (1767). 



Viewing the question anatomically, it is by no means certain that the rods 

 of Corti are so attached and stretched that they are capable of separate and 

 individual vibrations. It has not been demonstrated that certain of these 

 rods vibrate under the influence of certain notes or that they are tuned in 

 accord with certain notes. Hensen and others have rejected the theory of 

 Helmholtz, basing their opinions mainly upon the anatomical arrangement 

 of the rods of Corti. Hensen assumed it to be a physical impossibility for 

 the different rods to vibrate individually, and he regarded it as improbable 

 that the rods are tuned in accord with different musical notes. Similar ob- 

 jections apply to the theory that different transverse fibres in the membrana 

 basilaris vibrate in accord with particular notes. There is, indeed, no theory 

 which affords an entirely satisfactory explanation of the mechanism of the 

 final appreciation of the pitch of musical sounds. 



It is not absolutely necessary that sonorous vibrations should pass to the 

 cochlea through the external ear and parts in the middle ear. Sounds may 

 be conducted to the auditory nerves through the bones of the head or through 

 the Eustachian tube, as is shown by the simple and familiar experiment of 

 placing a tuning-fork in contact with the head or between the teeth, the ears 

 being closed. 



The action of the two ears does not seem to be absolutely necessary to the 

 correct appreciation of auditory impressions ; but variations in the force of 

 such impressions, made upon either ear, aid in determining the direction of 

 sounds, although errors are often made in this regard. 



The estimate of the distance of sounds is made by judging of the intensity, 

 in connection with information obtained through other senses, especially the 

 sense of sight. The power of estimating distance is largely influenced by 

 experience and education. 



Centres for Audition. The centres for audition in dogs and monkeys 

 are in the superior temporo-sphenoidal convolution (Ferrier, Munk). In 

 man these centres are in the first (superior) and second temporal convolu- 

 tions of the temporo-sphenoidal lobe, which are supplied by the fourth branch 

 of the middle cerebral artery. This has been ascertained by pathological 

 observations as well as by experiments on the lower animals. In man the 

 action of these centres is not completely crossed, and destruction of the cen- 

 tre upon one side does not cause complete deafness in either ear. Complete 

 destruction of the centres on both sides, however, produces total deafness. 

 Injury of the first temporal convolution is often followed by the condition 

 known as word-deafness, in which the subject hears the sound of words, but 

 these sounds convey to him no idea. This is the psychical, auditory centre, 



