CHAP, ix.j NERVOUS SYSTEM AND ORGANS OF SENSE. 



295 



upon which the image is thrown, is the retina, and the perfection of 

 the image is increased by the iris, which acts like any other 

 diaphragm by moderating the light and cutting off marginal rays, 

 the effect of which, if admitted, would be to produce the imperfection 

 called spherical aberration. The organic fibres of the iris spon- 

 taneously act to exclude or admit light through the pupil, which 

 dilates in obscurity and contracts in bright light, in the manner 

 already described. 



The different densities of the different media through which the 

 light has to pass in traversing the eye corrects another kind of 

 imperfection due to the scattering of colour, and renders it an 

 achromatic instrument. 



Not only is the image in the eye inverted, but each eye sees any 

 object from a slightly different point of view. It is this grasp, as it 

 were, of an object on both sides by the two eyes, which gives rise to 

 the apprehension of solidity * and relief. 



For distinct vision, the rays of light must be brought to a focus 

 on the retina. Now, without moving the body, neck, or head, the 

 eyes can be so directed alternately to nearer or to more distant 

 objects as to produce distinct perception of each successively. Such 

 movement is the focussing of rays coming from points varying in 

 remoteness. The way in which this adjustment is brought about is 

 supposed to be by the action of the ciliary muscle, and the physical 

 properties of the lens. The latter is highly elastic, and tends to be 

 more convex in front than is ordinarily the case. The ciliary 

 muscle, pulling the choroid, relaxes the ciliary ligament, and 

 therefore the pressure of the capsule on its contained lens, which 

 immediately becomes more convex, and so becomes accommodated 

 to the perception of nearer objects. f 



26. The ORGAN OF HEARING is divisible into three parts : the 

 external, the middle, and the internal ear. 



The EXTERNAL EAR consists of a cartilaginous, membranous, and 

 muscular structure, projecting conspicuously upwards from the side 

 of the head, and called the pinna or auricle, together with the 

 passage leading inwards from the deeply situated lowest portion of 

 the pinna, which passage is the external meatuSj or tube leading to 

 the internal ear. 



The PINNA has the form of the wall of a cone, cut obliquely down- 

 wards and outwards from its apex, the section being curved a little 

 upwards at the last, so as to make the lowest part of this conic section 

 almost semicircular. The pinna stands up above and behind the 



* It is the special artificial imitation 

 of two such views placed side by side at 

 a suitable distance, which produces the 

 illusion of the stereoscope, the relation of 

 each view to its proper eye being main- 

 tained. If the views be transposed, so 

 that the view which would naturally 

 present itself to the right eye be pre- 

 sented to the left, and vice versa, as by a 



jjseudoscopc, then the opposite appearance 

 of a hollow or excavated surface is pro- 

 duced. 



t The above is the commonly received 

 doctrine on the subject ; but fishes have 

 a ciliary muscle, although the lens in 

 them is globular. It may be, therefore, 

 that its action is not yet correctly under- 

 stood. 



