VISUAL ANGLES AND FIELDS 



297 



totally unrelated and unique habit, that of manipulation, accounts for 

 the development of frontality by the primates as we shall see later. 



The total visual field of mammals varies with the attitude of the optic 

 axes, from 360 in some rodents through 250 in the dog, to 180° in a 

 man whose eyes are in the position of rest. The situation in the horse has 

 been studied with particular care. Here, the temporal boundary of the 

 visual field runs backward parallel to the axis of the body, so that the 

 posterior blind area is not angular and constantly widening with increas- 

 ing distance. Thus the horse — when he holds his head up — cannot be 

 approached unawares from behind by any object bigger than his own 

 head. Anteriorly, the limits of the two monocular fields each cross the 



Fig. 113 — The angle between the optic axis and the body axis in various mammals. Re- 

 drawn, modified, from Lindsay Johnson. Families and sample species are shown on the 

 right side of the chart, larger taxonomic categories on the left. 



body axis and make 35.5° angles therewith, thus giving the horse a 71 

 binocular field together with nearly complete periscopy. Each eye sees 

 through an angle of 215°, which is probably a record unless it is exceed- 

 ed in some of the fishes. Some of the special devices which make possible 

 this wide monocular visual angle in the horse will be mentioned shortly. 

 According to Kahmann, mammals in general possess binocular fields 

 ranging in width from 20° (or less) to 40° in the rodents, to 120° or 

 more in the cats and prosimians and a maximum of 140° in the simians 

 and man. The ungulates are intermediate with values from 60° to 80°, 



