80 MANUAL OF ELEMENTARY ZOOLOGY 



which when it is slight escapes attention, but when it 

 becomes excessive rises into pain. Each of 

 ce" 88 mt a t" 8: t ' iese senses has origin in impulses derived 

 ments. * from a special kind of nerve ending, but only 

 in the case of sight, hearing, and smell are 

 these endings situated in a highly specialised organ. We 

 shall here consider only these organs. 



The eyeball of the frog is roughly spherical, but 



flattened on the front side. It consists of the following 



tr parts: (i) The outer 



coat or sense 

 capsule cor- 

 responds to the auditory 

 and nasal capsules, but 

 fits closely to the eye 

 instead of forming a 

 hollow capsule fused to 

 the skull. Over the 

 greater part of the eye 

 it consists of dense con- 

 h ' s ' cl " nective tissue with some 



FIG. 42.— The labyrinth of the right ear car tilage and is known 

 of the frog, seen from the outer side. „. t i r . • • 



-Partly liter Marshall. " l r he Orotic, but on 



. . ... . • , , . the front side it is trans- 



it* .s., Anterior vertical semicircular canal ; 



amp., ampulla:; chl, small dilatations of parent and knOWn aS the 

 the sacculus which represent the cochlea rnr „,„ /,\ -pL.- _i_- 



of higher animals; A.S., horizontal semi- Cornea. \2) I ne SKin 

 circular canal; »., branches of the audit *y over the COfflea adheres 

 nerve to supply the ampullae; p.v.s., pos- f : f j y . 



terior vertical semicircular canal ; iac. t tO It aS a GeilCate, tranS- 



saccuius ; utr., utricuius. parent covering, the con- 



junctiva, which is kept 

 moist by the secretion of Harderian glands below the 

 eye. (3) Inside the sense capsule is the choroid coat, 

 consisting of looser and highly vascular connective tissue 

 containing numerous dark pigment cells. In front the 

 choroid separates from the sclerotic and passes inwards, 

 as a partition called the iris, across the hollow of the 

 eyeball, which it thus divides into anterior and posterior 

 chambers. The former is smaller and filled with a watery 

 aqueous humour, the latter larger and filled with a gelatinous 

 vitreous humour. In the middle of the iris is an opening, 

 the pupil, and the iris contains muscular tissue by which 



