THE FIELD OF VISION 41 



'' pterygium ' -affections which are not met with in other 

 mammals than man. 



Wood Jones 1 has described how the changed relations 

 between the long axis of the skull and face and the long 

 axis of the vertebral column, which is produced in monkeys 

 as the result of arboreal life from the partial assumption of 

 the upright posture, has resulted in a changed position of 

 the occipital condyles on the base of the skull, which hinge 

 it to the vertebral column. In quadrupeds these condyles 

 are situated at the posterior extremity of the skull. In 

 most monkeys they are situated well forward on the base; 

 in anthropoids still farther forward; in man, who has assumed 

 the permanently erect posture, the head is practically 

 balanced upon the first cervical vertebra. 



As the head becomes more nearly balanced on the verte- 

 bral column, so the powerful ligamentous and muscular 

 attachments which pass between its posterior surface and 

 the cervical spinous processes become reduced in strength, 

 and the spinous processes themselves become reduced in 

 size. The even balancing of the head on the spine, and the 

 reduction in size of the cervical spinous processes, both 

 tend to facilitate the range and rapidity of the lateral and 

 rotatory movements of the head. Such increased move- 

 ments of the head, like the increased movements of the 

 eyes, serve to increase the area of the fields of fixation. 



