24 SPECIAL SENSES. 



SECTION II. 



OF THE SPECIAL SENSES. 

 1. Of Sight. 



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

 by means of which, the form, dimensions, position, color 

 and brilliancy of surrounding objects, are discerned. Some 

 of these properties maybe also ascertained, though in a less 

 perfect manner, by the sense of touch. We may obtain an 

 idea of the size and shape of an object, by handling it ; but 

 the properties that have a relation to light, such as color and 

 brilliancy, and also the form and size of borh^s that are be- 

 yond our reach, are exclusively recognized by sight. 



70. 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 are always in connection with particu- 

 lar nerves, called the optic nerves (Fig. 13, .). In the ver- 

 tebrates, these constitute the second pair of the cerebral 

 nerves, and arise directly from the middle mass of the 

 brain (Fig. 21, Z>), which, in the embryo, is the most con- 

 siderable of all. 



71. Throughout the whole series of vertebrate animals, 

 the eyes are only two in num- 

 ber, and occupy bony cavities 



of the skull, called the orbits. 

 The organ is a globe or hollow 

 sphere, formed by three mem- 

 branes enclosed one within the 

 other, and filled with transpa- 

 rent matter. Figure 13 repre- 

 sents a vertical section through Fi 

 the eye, and will give an idea of the relative position of 

 these different parts. 



