186 THE FROG CHAP 



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

 by injury to the retina or actual organ of sight, by cut- 

 ting 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, down- 

 wards, forwards, and backwards. And, finally, two 

 oblique muscles, the superior and inferior, produce a rota- 

 tion 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 secre- 

 tion 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 (which also serves for equili- 

 bration), like the organ of sight, consists of an essential 

 portion and an accessory apparatus. The essential 

 organ of hearing is a structure called the membranous 

 labyrinth, contained within the auditory capsule of the 



