322 Elementary Biology. 



been referred to (p. 267). Vibrations of the air cause the 

 membrana tympani to tremble in unison, and those trem- 

 blings are, by means of the columella and the membrane 

 covering the fenestra ovalis, communicated to the lymph 

 filling the inner ear, which in its turn stimulates the epithelial 

 cells in connection with the terminations of the auditory 

 nerve. 



Sight. The peripheral terminations of the nerve of sight 

 are more complicated than those of the other sense-organs. 

 The eye consists of a strong capsule of fibrous tissue, known 

 posteriorly as the sclerotic, anteriorly as the cornea. The 

 cornea is transparent, the sclerotic is white and opaque. . 

 The capsule is pierced behind by the optic nerve, and is 

 kept in its socket and at the same time moved by six bands 

 of muscle, four of which spring from the upper, under, and 

 two lateral margins of the eye, whilst the remaining two are 

 obliquely placed, one arising from the upper and outer 

 margin, the other from the under and inner margin. The 

 four muscles first mentioned are known as the superior, 

 inferior, exterior, and interior recti, while the oblique 

 muscles are spoken of as the superior and inferior oblique 

 respectively. It will be remembered that no less than three 

 out of the ten pairs of cranial nerves were distributed to the 

 muscles of the eye. Their distribution is as follows : 

 the superior, inferior, and interior recti, and inferior oblique 

 are supplied by the oculi-motor nerve (in.) ; the exterior rectus 

 is supplied by the abducens (vi.) ; while the pathetic (iv.) nerve 

 goes to the superior oblique muscle. Within the fibrous 

 capsule there are two chambers, a large posterior and a 

 small anterior. Lining the inner surface of the sclerotic is 

 a highly vascular pigmented membrane, the choroid, which 

 separates away from the sclerotic just where the sclerotic 

 becomes continuous with the cornea. It there forms a 

 diaphragm, ramed the iris, crossing the eye from side to 

 side, incomplete, however, in the centre, where a rounded 

 aperture is left, the pupil. Behind the iris lies a muscular 



