538 THE NERVOUS SYSTEM. 



of the sensations produced. The principal conditions regulating the 

 sense of sight are the following : 



Field of Vision. As the eyeball is placed in the orbit with the 

 cornea and pupil directed forward, there is, in front of each eye, a cir- 

 cular space within which objects are perceptible ; while beyond its bor- 

 ders nothing can be seen. This space is the "field of vision." Its 

 extreme limit, in man, reaches nearly 180 degrees of angular distance ; 

 that is to say, the light from a brilliant object may be perceived, when 

 the object is in a lateral position, almost as far back as the plane of the 

 iris. The possibility for a ray of light from this source, to penetrate the 

 pupil and reach the retina, depends on the refractive power of the cornea 

 and the curvature of its anterior surface, by which the ray is bent 

 inward and enabled to enter the pupil in an oblique direction. In many 

 animals, where the eyes are more prominent than in man, and the curva- 

 tures of the cornea and crystalline lens more pronounced, the field of 

 vision is enlarged in a corresponding degree. In birds and fishes, it is 

 still further modified by the lateral position of the eyes. The ostrich, 

 with the head directed forward, can easily see objects a few yards 

 behind its back ; and in many fish, when examined from different points 

 in an aquarium, it is impossible for the observer to place himself in any 

 position, above, behind, or on either side, where he cannot see one or 

 both of the pupils of the animal. The field of vision consequently, 

 for such animals, is a complete sphere ; the light being perceptible from 

 every point of the surrounding space. In man, the outer borders of 

 the field of vision are ill defined ; and objects at a lateral distance of 90 

 degrees must be very brilliant to attract attention. For practical pur- 

 poses, the space within which objects are perceptible is not more than 

 75 degrees on each side, or 1 50 degrees for the entire field of vision. 



Line of Direct Vision. Within the field of vision there is only one 

 point, at its centre, where objects can be perceived with distinctness ; 

 and the prolongation of this point, in the visual axis, is called the " line 

 of direct vision." Objects upon this line can be distinctly seen; all 

 others, situated on either side, above or below it, are perceived only 

 in an imperfect manner. If the observer place himself in front of a 

 row of vertical stakes, he can see those directly before the eye with 

 perfect distinctness ; but those on either side appear as uncertain and 

 confused images. On looking at the middle of a printed page, in the 

 line of direct vision, we see the distinct outlines of the letters ; while 

 at successive distances from this point, the eye remaining fixed, we 

 distinguish first only the separate letters with confused outlines, then 

 only the words, and lastly only the lines and spaces. 



This limitation of serviceable sight to the line of direct vision is 

 compensated by the mobility of the eyeball, which turns successively 

 in different directions ; thus shifting the field of vision and examining 

 in turn every point attainable by the eye. In reading a printed page, 

 the eye follows the lines from left to right, seeing each letter and word 



