1 86 THE FROG CHAP. 



As with the other sense-organs, sight may be destroyed 

 by injury to the retina or actual organ of sight, by cutting 

 the optic nerve, or by destroying the brain. But unlike the 

 other sense-organs already considered, that of sight has a 

 complex accessory or focussing apparatus in connection 

 with it, and vision may also be rendered impossible by 

 injury to the cornea or lens. 



It is an obvious advantage to an organ of sight such as 

 the frog's that it should be capable of movement in any 

 direction, so as to allow the light from any object to enter 

 the pupil. As a matter of fact, the animal can direct its 

 gaze through a very wide range by means of eight muscles 

 connected with the eyeball in the orbit. One of these, the 

 levator bulbi, raises the whole eye, causing it to project further 

 on the surface of the head. Another, the retractor bulbi, 

 withdraws it, causing it to bulge into the mouth. Four 

 others (compare p. 164 and Fig. 126), the superior, inferior, 

 anterior, and posterior recti, rotate it respectively upwards, 

 downwards, forwards, and backwards. And finally, two oblique 

 muscles, the superior and inferior, produce a rotation along 

 an axis joining the optic nerve with the middle of the cornea. 



The conjunctiva, which covers the outer surface of the eye 

 and lines the eyelids, is kept moist by the secretion of a 

 lacrymal gland, known as the Harderian gland, situated 

 between the eyeball and the orbit in the antero-ventral 

 region. The excess of this secretion is carried away into 

 the olfactory chamber by means of a tube, the naso- 

 lacrymal duct. 



Each organ of hearing, like that of sight, consists of an 

 essential portion and an accessory apparatus. The essential 

 organ of hearing is a structure called the membranous laby- 

 rinth, contained within the auditory capsule of the skull 

 (Fig. 10), and consisting of a kind of bag of very peculiar 



