Spatially Determined Reactions 165 



which the nerves supplying both ears had been cut recovered 

 after two or three weeks and could swim quite normally 

 except when they were placed in a large body of water and 

 made to swim rapidly, when they showed no power of pre- 

 serving their balance (33). Their successful performance 

 of slower movements was very likely due to the use of 

 sight. 



Sensory impulses from the body muscles themselves un- 

 doubtedly cooperate with those from the semicircular canals 

 in the maintenance of balance. They are evidently involved 

 in the peculiar withdrawing movements by which land ani- 

 mals, even puppies, kittens, and young rats whose eyes have 

 not opened, save themselves from falling when they reach 

 the edge of the object on which they have been crawling 

 (271, 384). Water-dwelling animals, accustomed to plunge 

 off solid supports, lack this protective instinct; Yerkes 

 showed that among several species of tortoises, some land- 

 dwelling, some amphibious, and some aquatic, the first men- 

 tioned were much more reluctant than the second to crawl 

 off the edge of a board, and the second more reluctant than 

 the third (459). 



65. The Psychic Aspect of Orientation to Gravity 

 Glancing back over these examples of the responses made 

 by animals to gravity, we note that while in some cases the 

 earth's attraction appears to act mechanically upon the 

 animal, causing the body passively to assume a certain 

 position, the common method of bringing about orientation 

 seems to be that some structure in the body, placed in an 

 abnormal position, presents a stimulus which brings about 

 a compensatory movement. This structure may be heavier 

 particles of the body substance, as probably is the case in 

 Paramecium ; it may be a statolith, or the fluid in the laby- 



