THE VISUAL AREA. 



757 



and that the movements take the direction towards which those 

 sensations are projected, then the above facts, especially if taken in con- 

 nection with the results of destructive lesions of the area, indicate a 

 certain connection between the parts of the cerebral visual area and the 

 retinae. This connection may be stated as follows : 



1. The whole of the visual area of one hemisphere is connected with 

 the corresponding lateral half of both retinae. 



2. The upper zone of the visual area of one hemisphere is connected 

 with the upper zone of the corresponding lateral half of both retinae. 



3. The lower zone of the visual area of one hemisphere is connected 

 with the lower zone of the corresponding lateral half of both retinae. 



4. The intermediate zone of the visual area is connected with the 

 middle zone of the corresponding lateral half of both retinae. 



5. The focal point of the visual area, which is placed on the anterior 

 part of the mesial surface of the occipital lobe, is connected with rather 

 more than the corresponding half of the macula lutea of each retina. 



These probable connections, in so far as they can be shown in a horizontal 

 plane, are illustrated in the appended diagram (Fig. 346). They can be easily 

 understood, if we imagine the visual 

 areas of the two hemispheres to be joined 

 together (with a little overlapping) at 

 their mesial surface to form a single hemi- 

 spherical cup of the size of the retina, 

 and if we then imagine the two retinae to 

 be placed one inside the other in their 

 natural position in contact with the cere- 

 bral cup thus formed. All correspond- 

 ing points, both of the two retinae and 

 of the cerebral visual area, will be in 

 contact with one another (Fig. 347). 



If we compare the scheme thus repre- 

 sented with that which is given by H. 



Munk for the dog (Fig. 345), we observe that the main point of difference is 

 that Munk locates the focal point of each visual area at the apex of the occipital 

 lobe. In accordance with this, he finds that destruction of this point on both 

 sides causes loss of central vision. Whether this is the case with dogs or not I 

 cannot certainly say, but my experiments upon monkeys do not agree with this 

 conclusion, nor does it find support from observations upon man. On the other 

 hand, there is abundant clinical evidence to show 



1. That lesions of the occipital lobe produce disturbances of vision which 

 are invariably of a hemiopic character. 



2. That lesions of the mesial surface of the lobe in the immediate neigh- 

 bourhood of the calcarine fissure are those which for their size produce the 

 most serious visual disturbances. On the other hand, lesions of parts near to 

 but not involving these parts of the occipital lobe have not infrequently been 

 associated with defective appreciation of visual objects, and particularly with 

 word-blindness. 



3. Perimetric observations in man show that in cases of hemiopia, produced 

 by lesions of the cortex (of one hemisphere), the line of demarcation between 

 the blind and functional parts of the retina usually passes, not through the 

 middle of the f ovea but on the homonymous side ; in other words, the fovea is 

 not involved in the hemiopic condition. 



The visual area in man appears to be relatively less extensive than 

 in the monkey. Seguin found that in by far the greater number of 

 cases of hemiopia examined, the cortical lesion was in the neighbourhood 



FIG. 347. Diagram to indicate the corre- 

 sponding points of the two retinae and 

 of the two halves of the visual cortex. 



