THE RABBIT. 115 



swing across the tympanum, from the tympanic membrane 

 to the internal ear. At two points the bony investment 

 of this last is incomplete at the fenestra rotunda, and at 

 the fenestra ovaliSj into which latter the end of the stapes 

 fits, and so communicates the sound- vibrations of the 

 tympanic membrane to the endolymph. A passage, the 

 Eustachian tube, communicates between the tympanic 

 cavity and the posterior narial pa'ssage, and serves to 

 equalize the pressure on either side of the drum-head. 



7. The Sense of Direction. So far we have spoken of 

 the ear as the organ of hearing simply; but the two parts 

 of the labyrinth really have two different functions. It 

 is only the sensory cells of the cochlea which are stimulated 

 by the vibrations transmitted to the endolymph by the 

 auditory ossicles. The sensory cells of the ampullae are 

 also stimulated by the varying pressure of the endolymph, 

 but this varies according to the position of the three canals 

 relative to the direction of gravitation. Hence the semi- 

 circular-canal system serves as an organ of direction, and 

 is of the greatest importance in connexion with the main- 

 tenance of equilibrium, as we shall see presently ( 12). 

 The arrangement of the canals in three planes mutually 

 at right angles is a means by which endolymph-pressure is 

 resolved into three components which can be separately 

 appreciated by the brain. The giddiness which results 

 from violently whirling oneself round is due to the rush of 

 endolymph causing abnormal pressure on the sensory cells 

 of the ampullae, and so utterly disturbing the sense of 

 direction. In birds, whose wonderful powers of flight and 

 migration involve an extraordinary sense of direction, the 

 canals are highly developed and very nearly circular. 



8. The Eye (fig. 62) has a tough outer coat of white 

 fibrous connective tissue, the sclerotic, within which is a 

 highly vascular layer, the choroid, within which comes the 

 essential, sensory layer, the retina. The chamber enclosed 

 by these three layers is filled with a transparent jelly, the 

 vitreous humour. In front, the white sclerotic passes into 

 the transparent cornea. The epidermis is continued over 



