174 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



the eye, and continued into a partition, the iris, by which the 

 cavity of the eye is divided into a small outer and a large 

 inner chamber. The iris is perforated by an oval slit (the 

 pupil) for the transmission of light. The innermost coat of the 

 eye is a delicate membrane, the retina, which contains the end- 

 organs for sight (rods and cones). The optic nerve perforates 

 the sclerotic and choroid on the inner side of the eye and 

 branches out in the retina, its fibres becoming indirectly con- 

 nected with the rods and cones. The interior of the eye is 

 occupied by refracting structures, by which the light is focussed 

 on the sensitive retina. These are : a watery fluid (aqueous 

 humour) external to the iris, a jelly-like substance (vitreous 

 humour} occupying the internal chamber, and a spherical trans- 

 parent lens suspended on the inner side of the iris. 



The eyelids have already been mentioned (p. 155). Other 

 important accessory parts are the six small band-like muscles 

 by which the eyeball is moved. Four of these take origin from 

 the hinder part of the orbit, and are inserted into the upper, 

 lower, anterior, and posterior sides of the eyeball, being respec- 

 tively known as superior, inferior, internal, and external red us 

 muscles. The other two are known as superior and inferior 

 oblique muscles, taking origin from the front end of the orbit, 

 and inserted respectively into the upper and lower sides of the 

 eyeball. 



CHAPTER X. AMPHIBIA. 

 15. RANA (The Frog). 



THE two commonest kinds of Frog are Rana temporaria, the 

 Common Frog, and Rana escul&nta, the Edible Frog. Both are 

 found on the Continent, but only the former in this country. 

 The following description applies to both, any important dif- 

 ferences being noted : 



