THE PERCEPTION OF SPACE 667 



of coordinates. Plant life orientates itself with regard to gravity 

 (geotropism) ; equally, in animal life the mechanism which maintains 

 the posture of the body — the basis of its perception of space — uses the 

 same fundamental reference-frame, and when a vertebrate is at rest it 

 utilizes an elaborate system of static postural reflexes designed to 

 maintain its equilibrium and provide a starting-point for its contacts 

 with the outside world. These are supj^lemented by a further system 

 of stato-kinetic reflexes which serve a similar jDurpose to the animal 

 in motion (Sherrington, 1904-6 ; Magnus, 1924). It is this combina- 

 tion which maintains the organism right-side-up and allows it to 

 retain its relationships with its environment. 



With this mechanism as basis, space is explored egocentrically by 

 the various senses, every one of which contributes in some degree to 

 the common aim. The immediate vicinity can be explored by the 

 tactile sense ; but the development of projicient senses is necessary 

 for the appreciation of anything beyond the restricted area which the 

 animal can touch. It is true that the tactile sense can be projected 

 to some extent, as by the appreciation of vibrations as is seen in the 

 ability of snakes to record ground-vibrations through the lower jaw,^ 

 a facility akin to that displayed by web-spinning and ripple -spiders. ^ 

 The olfactory sense and to a greater extent the auditory sense act as 

 adjuvants for this purpose, but with the exception of the astonishing 

 development of the auditory powers of bats,'^ these form inefficient and 

 unreliable guides. The remarkable thermal sense of certain colubrid 

 snakes ^ forms another exception ; but as a general rule throughout 

 the vertebrate phylum extended spatial judgments, at least in diurnal 

 species, are essentially dependent on vision which makes by far the 

 greatest contribution to the perceptual range of the animal and to 

 the accuracy of its assessments. 



Visual spatial perceptions may be classified into two main types 

 each of which may be divided into two attributes : 



(1) Bi-dimensional perceptions, made up of 



[a] the perception of direction which allows an estimation to be 

 made of the position of an object relative to the body ; 

 and (6), an extension of this faculty into the perception of bi- 

 dimensional distance (or extensity) allowing an estimate to 

 be made of the angular extension of an object. 



(2) Tri -dimensional perceptions, made up of 



(a) the perception of depth which includes the capacity of stereo- 

 scopic vision when binocularity is attained; 

 and (6), an extension of this faculty into the perception of size, a 

 psychological appreciation of the size of a body emerging as 



1 p. 599. 2 p_ 579. 



3 p. 601. * p. 600. 



