282 THE LOWER PRIMATES 



skilled acts pertain to those performances demanding the greatest manual 

 dexterity, such, for example, as handwriting, carving, painting and the use of 

 instrimients, implements and tools. Many skilled performaiices of a much 

 simpler nature are also dependent upon vision for their acquisition and reten- 

 tion. This fact may be discerned in the infant learning to grasp, hold and 

 manipulate objects. It is, however, only as these manual activities become 

 more complex, more exacting in their precision, that the full degree of visual 

 regulation is demanded by them. The advantage of stereoscopic vision in 

 producing perspective and proper distance relations then becomes indispens- 

 able. In other words, stereoscopic vision should be regarded as a fundamen- 

 tal contribution to the upbuilding of the most highly complex performances. 

 Even in the lower primates this tendency is clearly observed, as the coeffi- 

 cients of the oculomotor decussation show. Thus, in the lemur, whose eyes 

 are widely separated and in which the need of extensive stereoscopic vision 

 is not pronounced, the oculomotor decussation occupies but i6 per cent of 

 the entire oculomotor nucleus. Only a relatively small portion of this aggre- 

 gation of cells is brought into close internuclear relation. This fact accords 

 with the relatively limited degree of skilled performances of which these 

 animals are capable. In tarsius it was impossible to estimate the extent of 

 the decussation by mensuration. The animal possesses a certain degree of 

 binocular vision which, however, is probably not stereoscopic. 



Marmoset, on the other hand, shows a more extensive internuclear con- 

 nection. Its longitudinal coefficient of the oculomotor decussation is 38 per 

 cent, a fact which is quite in keeping with the closer relation of the eyes 

 and a greater range of hner movements. It would seem, thereiore, in this 

 comparison, that binocular vision is more essential to the marmoset than to 

 the lemur. 



A still more striking ditfercnce is apparent in the mycetes, the oculo- 

 motor decussation of which occupies 69 per cent of the entire length of the 



