320 THE SPECIAL SENSES. 



the pigment moves outwardly and collects around the external 

 segments, where the process of regeneration of the visual purple is 

 taking place. Further evidence that the visual purple is connected 

 with the irritability of the rods toward light stimulation is shown 

 by the fact that when it is exposed to the different rays of the spec- 

 trum the absorption of light is greatest in that part of the spec- 

 trum (green) which appears the brightest in vision when carried out 

 under such conditions as may be supposed to involve the activity 

 chiefly of the rods (see below for these conditions). It is, however, 

 perfectly obvious that visual purple is not essential to vision. The 

 fact that it is absent from the fovea centralis is alone sufficient 

 proof of this statement. Moreover, it seems to be absent entirely 

 in the eyes of some animals; for instance, the pigeon, hen, some 

 reptiles, and some bats. The most attractive view of the function 

 of the visual purple is that it serves to increase the delicacy of re- 

 sponse or irritability of the rods in dim lights, a view that is ex- 

 plained in more detail in the paragraph below, dealing with the sup- 

 posed difference in function between the rods and cones. 



The Extent of the Visual Field Perimetry. By the visual field 

 of each eye is meant the entire extent of the external world which 

 when the eye is fixed forms an image upon or is projected upon the 

 retina of that eye. From what has been said previously regarding 

 the dioptrics of the eye it is obvious that the visual field is inverted 

 upon the retina, and that, therefore, objects in the upper visual field 

 fall upon the lower half of the retina, and objects in the right half 

 of the visual field fall upon the left half of the retina. Since the 

 retina is sensitive to light up to the ora serrata, it is evident that if. 

 the eye were protruded sufficiently from its orbit its projected visual 

 field when represented upon a flat surface would have the form of a 

 circle the center of which would correspond to the fovea centralis. 

 As a matter of fact, the configuration of the face is such as to cut 

 off a considerable part of this field and to give to the field as it 

 actually exists an irregular outline. The bridge of the nose, the 

 projecting eyebrows and cheek bones serve to thus limit the field. 

 To obtain the exact outline and extent of the visual field in any given 

 case it is only necessary to keep the eye fixed and then to move a 

 small object in the different meridians and at the same distance 

 from the eye. The limits of vision may be obtained in this way 

 along each meridian and the results combined upon an appropriate 

 chart. An instrument, the perimeter, has been devised to facilitate 

 the process of charting the visual field. It has been given a number 

 of different forms, one of which is illustrated in Fig. 136. The shape 

 of the visual fields in the normal eye is represented in Fig. 137. 

 The determination of the visual fields is of especial importance in 

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



