THE PERCEPTION OF SPACE 669 



localization, the postural mechanism persists which extends the frame of 

 reference to provide a gravitational localization (Fig. 797). Visual 

 perceptions are synthesized with impressions from the extra-ocular 

 muscles, the neck and the labjTinths, so that visual orientations are 

 related to movements of the eyes with respect to the head, of the head 

 in respect to the trunk, and of the trunk in respect to gravity, and thus 

 an exploration of space is attained on a gravitational basis. These 

 fundamental mechanisms on the physiological level are irmate and 

 hereditarily transmitted, and upon them is erected a further psycho- 

 logical structure determined by the experience of each individual by 

 which the accuracy of spatial judgments is considerably increased and 

 their value to the animal augmented. 



It will be seen that the two fundamental coordinates are vertical and 

 horizontal in direction, the first determined gravitationally from the earth's 

 centre, the second visually from the horizon. We have already seen that the 

 visual responses of the octopus suggest that these directions have a special status 

 not only in the end-organ but also in its projections onto the primitive central 

 nervous system,^ a circumstance which indicates their phylogenetic age and 

 practical importance. 



Before discussing the part played by spatial perceptions in the 

 behaviour of Vertebrates, it wall be useful to discuss the basic physio- 

 logical factors which underlie such judgments — the mutual relation of 

 the visual fields, the occurrence of binocular as opposed to panoramic 

 vision, and finally the nature of reflex and voluntary ocular movements 

 and their relation to visual perceptions. 



THE UNIOCULAR VISUAL FIELDS OF VERTEBRATES 



The uniocular field of Vertebrates is relatively constant, averaging 

 in angular size about 170°. The estimation can be made theoretically 

 by optical calculation (Grossmaiui and Mayerhausen, 1877) or objec- 

 tively and more effectively by observing the image of a moving light 

 as seen by transillumination through the sclera, a method introduced 

 by the great physiologist, Johannes Miiller (1826), in his study of 

 corresponding retinal points, and applied to the determination of the 

 uniocular and binocular fields by Armin Tschermak (1902), Rochon- 

 Duvigneaud (1921-23), Verrier (1930), and others. Following Miiller's 

 lead, finictional confirmation of these results may be obtained in 

 animals which will respond suitably by the subjective method of noting 

 the angle at which an object will attract attention. 



The extent of the field varies essentially with three factors — the 

 angular extent of the retina, the curvature of the optical surface 

 admitting the light, and the effective pupillary aperture. The first of 

 these is the most important, and is relatively constant. Variations, 



^ p. r)76. 



