BINOCULAR VISION 589 



distance from the face, so that the eye axes are strongly converging and the 

 accommodation correctly adjusted to the same plane. The presence of a 

 reflex phase is well shown by the fact that no effort of the will is required 

 to sustain fixation on an object in which we are interested, and also by those 

 cases in which when once an object has been fixated, there is found to be 

 considerable mental difficulty in turning the gaze elsewhere. Rotation fixa- 

 tion on the other hand appears to be entirely reflex. In order that fixation 

 should be obtained when the gaze is directed in different directions, it is 

 necessary that there should be close association between the corresponding 

 muscles of the two eyes. This is at all events assisted by the anatomical 

 arrangement of the 3rd, 4th and 6th cranial nerve nuclei which has been 

 described previously (see page 496). 



Not only are the corresponding nuclei on the two sides connected by 

 transverse fibres so that e.g. either the superior recti or the inferior recti 

 move together, but the external rectus nucleus of one side is joined to the 

 internal rectus . nucleus of the other by the dorsal longitudinal bundle, so 

 that the eyes deviate together to right and left. Similar connections are 

 to be found between the nuclei of the superior and inferior obliques. The 

 relations of these nuclei to the cerebral cortex have been ascertained by elec- 

 trical stimulation. It has been found that stimulation of the median third of 

 the limb of the angular gyrus on either side causes both eyes to be turned 

 to the opposite side. The right gyrus therefore connects with the nuclei 

 of the right internal rectus (3rd) and the left external rectus (6th). 

 Since^ both these nuclei are on the left, the fibres from the gyri must cross 

 in order to reach their corresponding nuclei : this they do at the level of the 

 anterior corpora quadragemina. The angular gyri are connected to both 

 the frontal and occipital parts of the cortex, so that voluntary movements 

 of the eyes, and also movements under the action of light, can be carried out. 

 Experimental stimulation of the semicircular canals is found to cause 

 conjugate deviations of the eyes. But stimulation of the canals is effected 

 naturally by a rotation of the head, as will be described later. The conjugate 

 deviation of the eyes would appear to be initiated in order that the gaze 

 might remain stationary on external objects in spite of the head move- 

 ments. 



The way in which involuntary fixation is brought about may be described 

 as follows : when an image falls on the periphery of the retina an impulse 

 reaches the oculo-motor nuclei in the manner described above. Thus, 

 suppose the image to come from the right, it will fall initially on the left 

 halves of both retinae, and impulses will therefore travel to the left occipital 

 cortex. From here they will pass to the left angular gyrus, causing impulses 

 to travel to the left internal and the right external recti. Both eyes are 

 therefore directed to the right, the movement being such as to bring the 

 image on the fovea. But as the fovea is approached, the impression sent to 

 consciousness becomes increasingly distinct, owing to the higher acuity of 

 the fovea. If the fovea is passed the image begins to become indistinct 

 again, and therefore the movement of the eyes is checked as soon as the image 



