112 THE LOWER PRIMATES 



It has already been noted that the animal is able to turn its head com- 

 pletely around so that it may in eflcct look both forward and backward. 

 Such rotation is due to the fact that the animal moves its eyes relatively little 

 in the visual pursuit of objects. If it wishes to cover its visual field it does so 

 in great measure by movements of the head. This predominance of ccphalo- 

 gyric action over ocular movement may account for the extensive development 

 of the ventral gray column in those cervical regions of the spinal cord in 

 which the spinal accessory nerve takes origin. Thus may be explained the 

 preeminence of the neck muscles in acts of visual pursuit, as well as the incon- 

 siderable development of the inferior olive. From the behavior of tarsius it is 

 clear that the animal does not employ the muscles of the eyes, hands and 

 head in many acts requiring intimate cooperation. It is essentially a noctur- 

 nal hunter, and this fact in itself limits its manual activities to a somewhat 

 restricted range of performance. The remarkable dexterity of the tarsier in 

 capturing insects on the wing, as it is said to do, probably represents the acme 

 of its skilled achievement. This no doubt is a highly specialized act, but it is 

 no more complex than that manifested by many birds which capture their 

 prey while on the wing. Such development of skill would not necessarily make 

 great demand upon the olivary mechanism. Two other facts support the view 

 that tarsiers do not require particular exactitude in the simultaneous control of 

 the head, eye and hand muscles, i.e., the absence of retinal macula and the 

 more or less complete decussation of fibers in the optic chiasm. Both of these 

 important conditions imply the absence of stereoscopic vision, as they also 

 indicate the lack of those visual specializations essential to the development 

 of highly skilled acts. The indispensable relation of simultaneous coordina- 

 tion of head, eye and hand in such acts as handwriting is obvious. In acts of 

 this kind the eyes follow the hand, and the head cooperates with the ej^es in 

 this exact visual guidance. Tarsius is capable of no reactions comparable in 

 any sense to such a highly developed skilled performance. It manifests no 



