246 



ANATOMY OF VERTEBRATES. 



affords a non-reciprocating cavity for the free vibration of its 

 membrane and of the otosteals : it also renders the labyrinth 

 independent of atmospheric vicissitudes. The otosteals conduct 

 vibrations from the tympanic membrane to the vestibular one, 

 and, under the influence of the muscles, regulate the tension of 

 both these and of the cochlear fenestra, so as to protect the ear 

 against the effects of sounds of great intensity. The external 

 ear and meatus are collectors and conductors of vibrations, and 

 the former assists in enabling us to judge of the direction of 

 sounds. 



§217. Organ of Sight. — A. Eyeball. The organ of sight, 

 like that of smell, is wanting in a few Mammals, the eyeball 

 being reduced to the size and condition of the ' ocellus ' in Am- 

 blyopsis, and to its simple primitive office of taking cognisance 

 of light, a filament of the fifth aiding the remnant of a proper 

 optic nerve. The moles, especially the Italian kind, Talpa 

 cceca, and mole-rats, exemplify this condition, in which, as in 

 Spalax typhlus, the skin passes over the ocellus without any pal- 

 pebral opening, or loss of hair. The eyeballs are very small in 

 the allied genus Bathyergus, fig. 174, and other rodent bur- 

 rowers : they acquire the largest absolute and proportional size in 

 the Ruminant order. In no Mammal is bone developed in the 



sclerotic: in most a special ca- 

 vity, called ' orbit,' is fashioned 

 in the facial part of the skull to 

 give lodgment to the eye-ball. 

 One sees least indication of it 

 in the blind quadrupeds above 

 noted and in the ant-eaters : it 

 is deepest, best defined, and 

 most completely walled in Man. 

 In all Mammals with the eye 

 developed for sight, properly 

 so called, we recognise, as in the 

 diagrammatic section, fig. 193, 

 the fibrous capsule, a, called 

 e sclerotic coat,' the transparent 

 fore part, b, called c cornea ; ' the vascular tunic, <?, called c choroid 

 coat,' becoming thickened, at d, by the so-called ' ciliary ligament,' 

 from which the e ciliary processes ' are, as it were, reflected back- 

 ward upon the capsule of the lens,^/*: while the movable curtain, 

 or ( iris,' is continued onward into the space between b and /, 

 leaving a central opening, called f pupil,' for the admission of 



193 



Diagrammatic section of Mammalian eye. cv' 



