338 METAZOAN PHYLA 



tion; these vibrations are transmitted by the bones of the middle ear to 

 the fluid which fills the inner ear and by this fluid to the sensory cells in 

 the cochlea. 



The withdrawal of the inner ear from the surface and its lodgment in 

 a cavity in the skull are clearly in the interest of greater protection to an 

 organ which has acquired a more delicate adjustment in the higher forms. 

 The chain of ossicles forms a very sensitive sound-conducting apparatus. 

 The connection of the middle ear with the pharynx adjusts the pressure 

 in the middle ear to changing air pressures outside the body. As the 

 pressure of the surrounding air rises or falls, the tympanic membrane 

 would be forced inward or outward if it were not that through the eusta- 

 chian tube air enters or leaves the middle ear, thus equalizing the pres- 

 sures on the two sides of the membrane. Pressures are equalized as 

 between the middle and inner ear by the elasticity of the membrane 

 closing the opening in the wall of the sacculus. When the pressure in the 

 middle ear rises, this membrane is bent inward, and the pressure within 

 the inner ear is correspondingly increased; when the pressure in the mid- 

 dle ear falls, the membrane is bent outward, and the pressure in the inner 

 ear is correspondingly diminished. 



355. Eye. — The eye is the organ of sight and consists of a variety 

 of structures which contribute to this function. The eyeball, which is 

 the organ of sight proper, is somewhat like a roughly spherical camera 

 (Fig. 236). Its wall is composed of three layers. These are the outer, 

 or sclerotic, layer, which gives support; the middle, or choroid, layer, 

 which is vascular; and the inner, or retinal, layer, which is sensory. 

 Light is admitted through a transparent cornea, which is the anterior 

 portion of the sclerotic coat. The amount of light falling upon the sen- 

 sitive retina is regulated by a circular curtain, the iris, a part of the 

 choroid layer; the central opening in the iris is the pupil. Just behind 

 the iris is the lens, by means of which the rays of light, which have to a 

 degree been brought together by the convex cornea, are focused on the 

 retina. When light stimuli fall on the retina, they give rise to impulses 

 which are conveyed to the brain and there produce the sensation of sight. 

 Behind the retina is a pigment layer the function of which is not known. 

 The cavity of the eye is divided into two chambers. The outer chamber, 

 in front of the lens, is filled with watery aqueous humor; and the inner 

 chamber, behind the lens, is filled with the jelly-like vitreous body. The 

 outer chamber is again divided into the anterior chamber in front of the 

 iris and the posterior chamber behind it. The structure of the eye can 

 best be understood by reference to a diagram (Fig. 236). 



Accommodation, which is the adjustment of the eye to far and near 

 vision, is limited in the lower vertebrates but is well-developed in the 

 higher ones. It may be described as it occurs in man. It should first 

 be stated that the lens is elastic and tends of itself to become thicker 



