MAMMALS 



13 



light. The optic nerve enters at the posterior end of the eyeball. It 

 penetrates the two outer membranes (sclerotic and choroid), and then 

 expands in the form of the retina upon the inner side of the choroid. 

 Directly behind the iris is a lens-shaped body, the crystalline lens. The 

 space in front of the iris is filled with a watery fluid, the aqueous humour ,■ 

 that behind the lens with a gelatinous substance, the vitreous body. 

 Both the aqueous humour and vitreous body as well as the lens are 

 highly transparent. 



The eye is constructed on the same plan as the camera of the 

 photographer. On the retina, as on the sensitive plate of the 

 camera, there is formed an in- 

 verted and diminished image of the 

 external world, and the retina, being 

 composed of nerve terminations 

 sensitive to light, transmits the 

 image to the brain. (What is the 

 importance of eyelids, eyelashes and 

 eyebrows ?) 



(b) The ear is the organ of 

 hearing. In most mammals ex- 

 ternal funnel-shaped projections, 

 the pinnae, are provided for catching 

 the sound-waves. By the external 

 auditory aperture these waves enter 

 the ear passage, or external auditory 

 meatus, the end of which is closed 

 by a fine membrane, the tympanic 

 membrane. The sound-waves induce 

 vibrations in this membrane, which 

 are transmitted to the small bones 

 of the ear lying in the cavity of the 

 tympanum, which is filled with air. 

 A canal, the Eustachian tube, leads 



from the throat into the tympanic cavity, and thus establishes com- 

 munication with the external air. The end of the innermost of the 

 small ear-bones (stapes) is applied to a fine membrane of the inner ear, 

 or labyrinth, which lies in a corresponding bony cavity. This membrane 

 in its turn receives the vibrations and transmits them to a fluid contained 

 in the labyrinth. Thence the vibrations reach the terminations of the 

 auditory nerve, which expands within the labyrinth, whence they are 

 conveyed to the brain, where they enter into the consciousness in the 

 form of tones or noises. 



M. 



Human Eak. 



, External ear, or pinna; G., external 

 auditory meatus ; T. , tympanic membrane ; 

 P. , tympanic cavity ; G. K. , auditory ossicles ; 

 eu.R., Eustachian tube ; V., h.K. and Sch., 

 portions of the labyrinth ; N. , auditory 

 nerve. 



