SPECIAL SENSES. 2OI 



pani of the seventh, the posterior one-third, recognizing bitters 

 and sweets, being supplied by the glosso-pharyngeal nerve. The 

 circumvallate or calyciform papillae are probably gustatory, the 

 other papillae serving to entangle substances, so that their flavor 

 can be detected. 



HEARING. 



The External Ear collects the sound-waves and carries them 

 inward to the membrana tympani. 



The Middle Ear. 



The membrana tympani receives the sound-waves and transmits 

 them through the ossicles to the foramen ovale opening into the 

 vestibule. 



The Eustachian tube allows air to enter the tympanum, and 

 thus equalizes the pressure on both sides of the membrana tympani. 

 The guttural pouch, at its termination below, is probably connec- 

 ted with phonation. 



The Internal Ear. 



The semicircular canals have some function connected with 

 maintaining equilibrium. 



The organs of Corti are probably the essential parts of the in- 

 ternal ear, and here the sound-waves are recognized, each sound 

 by a separate organ, the organs receiving the waves by vibrations 

 of the endolymph transmitted from the perilymph. 



SIGHT. 



Sight is the special sense by which we are enabled to form an 

 idea of objects by means of the eye. 



Mechanism. 



The rays of light enter the eye by passing through the cornea, 

 aqueous humor, pupil, crystalline lens and vitreous to the retina, 

 where they are recognized and the impression sent to the brain 

 by the optic nerve. 



The function of the cornea, aqueous, and lens is to refract the 

 rays of light and bring them to a focus on the retina. 



The iris regulates the amount of light entering the eye by 

 means of its muscular fibres dilating and contracting the pupil. 



The retina receives the impression of light, the layer of rods 



14 



