792 VISUAL SENSATIONS. [BOOK m. 



must exist remarkably delicate and exact arrangements between 

 the two halves of the brain by which in each case as it occurs, the 

 two sets of visual sensations should be accurately pieced into a 

 whole perception in which we can subjectively recognize no 

 halves. It is at least strange that while the motor nervous mech- 

 anism for the two eyes is finely bilateral, the sensory mechanism 

 should be baldly unilateral. We need not then be surprised to 

 meet with other experimental results discordant with the above 

 mentioned apparently simple result. In the first place not only 

 is the hemiopia, caused by the removal of one occipital lobe, often 

 transient, but also, according to some observers, the lost vision 

 may return after the total removal of both lobes, though some 

 impairment may be noticed long afterwards, so long in fact as 

 the animal is kept alive. In the second place in the hands 

 of other observers destruction not of the occipital lobe but 

 of the angular gyrus of one side (Fig. 122) has led to hemi- 

 opia, failure in the left (or right) visual fields, indicating 

 failure in the central endings of the right (or left) optic 

 tract, being caused by removal of the right (or left) gyrus, 

 and destruction of both angular gyri has led to total blindness, 

 not only the hemiopia but the total blindness being, however, 

 apparently transitory. And cases have been observed in which 

 the blindness due to removal of the occipital lobe which would be 

 by itself transient has been made permanent by the subsequent 

 removal of the angular gyrus. In other cases again destruction 

 of one angular gyrus has produced, not hemiopia, but crossed 

 blindness or crossed amblyopia, that is to say has affected the 

 whole of the retina of one eye, and that the crossed eye, the eye 

 of the same side not being, or being supposed not to be, at all 

 affected ; and indeed similar results have also been stated to 

 follow upon removal of one occipital lobe. 



In man clinical histories so far conform to the results of ex- 

 periments on the monkey as to associate the occipital cortex, and 

 more particularly the cuneus (see Figs. 134, 135) with vision. 

 A very large and increasing number of cases have been recorded 

 in which hemiopia, a blotting out of the corresponding halves of 

 the visual fields of the two eyes (homonymous hemiopia as it is 

 called) has been associated with disease of the occipital lobe 

 namely of the cuneus, or of adjoining parts of the lingual lobe, 

 in the neighbourhood of the calcarine fissure, or of adjoining por- 

 tions of the occipital convolutions; and there have been similar 

 cases where not the half, but a part only, a quadrant it may be 

 or less, of each visual field has been blotted out. The teaching 

 of such cases is in full accord with the anatomical leading, but 

 leaves untouched the difficulty mentioned above as attaching to 

 such unilateral cerebral action. It is worthy of note that in many 

 such cases of hemiopia, the macula lutea, the region of distinct 

 vision is left intact in both eyes ; and this has led some to the 



