<18 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 outlines, dimensions, relative posi- 

 tion, color and brilliancy of objects are discerned. Some 

 of these properties may be 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 bodies that are be- 

 yond our reach, can be recognized by sight only. 



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 in all the higher animals are in connec- 

 tion with particular nerves, called the optic nerves, (Fig. 13, 

 a.) In the vertebrates, these are the second pair of the cer- 

 ebral 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 princi- 

 pal membranes, enclosed one 

 within the other, and filled with 

 transparent matter. Figure 13 

 re p resents a vertical section 



