238 THE SPECIAL SENSES 



to 70 seconds in the field of vision to be clearly seen. Such an 

 object makes an image TWDIF f an mcn on the retina, and this 

 is about the distance between the cones at the macula lutea. 

 In order that two points may be distinguished, they must be 

 separated at least this amount. 



Hearing. It may be accepted as a well-defined physiological 

 fact that the nervous structures of the cochlea form that organ 

 by which musical sound and noise of all kinds are converted 

 into nerve impulses. The sound waves of the air, which origi- 

 nate in vibrating bodies, are gathered together by the concha, 

 carried into the external auditory canal, and vibrate against 

 the membrana tympani. The latter takes up the vibrations 

 and transmits them through the chain of ossicles to the stapes 

 in the fenestra ovalis. The stapes imparts its motion to the 

 perilymph of the vestibule. There is now set up in the peri- 

 lymph a fluid wave that travels in all directions. It passes along 

 the scala vestibuli to the apex of the cochlea, then through the 

 aperture of communication with the scala tympani down the 

 latter until it expends itself against the membrane of the fenestra 

 rotunda. In its passage the fluid vibrates against the membrane 

 of Reissner and the basilar membrane, and this sets up similar 

 vibrations in the endolymph of the canalis cochlearis. The fluid 

 wave in the canalis cochlearis is in a position to irritate the 

 hair cells of the organ of Corti. These cells seem to be able to 

 respond to particular tones by their sensitiveness to certain 

 rates of vibration. But the fact that the organs of Corti are 

 absent in birds which evidently are capable of appreciating 

 musical tones shows that they are accessory and not absolutely 

 essential. 



The branch of the eighth nerve, having received its impulses 

 from the cells of the organ of Corti, transmits them to the centre 

 under the acoustic tubercle in the floor of the fourth ventricle; 

 thence fibers pass by means of the trapezium in the pons to 

 the opposite side, and through the lower fillet of that side to 

 the posterior quadrigeminal body, whence by means of the 

 brachium, internal geniculate body, optic thalamus, and internal 

 capsule, they proceed to the cortex of the superior temporal 

 convolutions. 



By subjective hearing is meant sounds that are heard distinctly 

 mid yet are not produced by physical sound waves from the 



