58 SPECIAL SENSES. 



SECTION II. 



OF THE SPECIAL SENSES. 



1. Of Sight. 



120. SIGHT is the sense by which light is perceived, and 

 by means of which, the outlines, dimensions, relative position, 

 colour, and brilliancy of objects are discerned. Some of these 

 properties may be also ascertained, though in a less perfect 

 manner, by the sense of touili. We may obtain an idea of 

 the size and shape of an obji-rf, by handling it ; but the pro- 

 perties that have a relation t-> light, such as colour and bril- 

 liancy, and also the form and size of bodies that are beyond 

 our reach, can be recognized by sight only. 



121. The EYE is the organ of vision. The number, struc- 

 ture, and position of the eyes in the body is considerably 

 varied in the different classes. But whatever may be their 

 position, these organs, in all the higher animals, are in con- 

 nection with particular nerves, called the optic nerves (fig. 13, 

 a). In the vertebrata, these are the second pair of the cerebral 

 nerves, and arise directly from the middle mass of the brain (fig. 

 20, b), which, in the embryo, is the most considerable of all. 

 122. Throughout the whole series of vertebrate animals, 



the eyes are only two in number, 

 and occupy bony cavities of the 

 skull, called the orbits. The 

 eye is a globe or hollow sphere, 

 formed by three principal mem- 

 branes enclosed one within the 

 other, and filled with transpa- 

 rent matter. Fig. 37 represents 

 a vertical section through the or- 

 gan, and will give an idea of the 

 relative position of these different 

 parts. 



1 23. The outer coat is called the sclerotic (b); it is a thick, 

 firm, white membrane, having its anterior portion transparent. 

 This transparent segment, which seems set in the opaque 

 portion, like a watch-glass in its rim, is called the cornea (/). 

 124. The inside of the sclerotic is lined by a thin, dark 

 coloured membrane, the choroid (c). It becomes detached 

 from the sclerotic when it reaches the edge of the cornea, 

 and forms a curtain behind it. This curtain give- to the eye 



