338 



THE SPECIAL SENSES. 



of the visual fields in the normal eye is represented in Fig. 144. 

 The determination of the visual fields is of especial importance in 

 cases of brain lesions involving the visual area in the occipital lobe. 

 The extent and portion of the retina affected may be used to aid in 

 locating the seat of the lesion. For physiological and for clinical 

 purposes it is necessary to distinguish between the central (or direct) 

 and the peripheral (or indirect) fields of vision. The former term 

 is meant to refer to that portion of the field which falls upon the 

 fovea centralis; in other words, it is the projection, in any fixed 

 position of the eye, of the fovea into the external world. The 

 peripheral field refers to the rest of the visual field which is pro- 

 jected upon the retina outside the fovea. As a matter of fact, all 

 of our distinct and most useful vision, in the daytime at least, is 

 effected through the fovea. When the eye is kept fixed, the small 

 portion of the external world that falls upon the fovea is seen dis- 

 tinctly. All the rest is seen more or less indistinctly in proportion 



Fig. 144. — Perimeter chart to show the field of vision for a right eye when kept in a fixed 

 position. 



to the distance of its retinal image from the fovea. In using our 

 eyes, therefore, we keep them continually in motion so as to bring 

 each object, as we pay especial attention to it, into the field of 

 central vision. The line from the fovea to the point looked at is 

 designated as the line of sight or visual axis. The area of the fovea 

 is quite small. The measurements given by different observers 

 vary somewhat, especially as in some cases the measurements are 



