SUMMARY OF STRUCTURES 281 



quently, the presence of a rich internuclear communication vouchsafes the 

 more accurate conjugation of the visual axes in the interest of liinocular 

 vision. 



The Nucleus of Perlia. The fate Dr. John Hunter has demonstrated 

 that a speciaf medial group of cetis, the nucleus of Perlia, develops especially 

 in tlie interest of ocular convergence. The nucleus is small or absent in ani- 

 mals possessing little or no attainment of binocular stereoscopic vision. It is 

 present in tarsius but in that species is much smaller than in the higher pri- 

 mates whose vision is effectively stereoscopic. The phyletic significance of 

 this nucleus of Perlia corresponds closely with that of the interoculomotor 

 decussation. 



EFFECT OF DEVELOPMENT OF STEREOSCOPIC VISION 



The development of stereoscopic vision has profound effects which far 

 exceed the limits of visual sensibility alone. It actually produces a decisive 

 influence upon the development of motion. It particularly affects those move- 

 ments which depend for their execution upon the fine, minute and precise 

 skilled acts employed in performances acquired and directed by vision. There 

 are certain acts which may be acquired only if vision is capable of directing 

 and furnishing the proper spatial relations for their guidance. Such a highly 

 skilled performance as taking aim at a distant mark could be acquired and 

 executed only with visual assistance. Acts in which a definite aim or direction 

 is a prerequisite, although the distance from the eyes may be but slight, also 

 require a similar degree of visual supervision. 



COMPARISON OF THE VISUAL FUNCTION OF THE LOWER PRIMATES 



A great number of skilled performances depend, therefore, upon the 

 function of vision. They are often rendered impossible or defective if for any 

 reason they are deprived of visual guidance. The largest number of these 



