THE VISUAL AREA. 



755 



area, the lower zone with the posterior part, the mesial or internal zone 

 with the mesial or internal part, and the lateral zone of the retina 

 with the lateral part of the area, and that the part of the retina 

 concerned with most distinct (central) vision is represented upon both 

 visual spheres over an area which lies near the apex of the occipital lobe 

 (in the monkey rather nearer the mesial surface) ; and he finds confir- 

 mation of these conclusions from the results of excitation performed in 

 conjunction with Obregia. 1 These results of Munk are embodied in 

 the diagram on the previous page (Fig. 345). I myself, in conjunction 

 with Sanger Brown, 2 found in the monkey that bilateral injury of both 

 mesial surfaces of the occipital lobes appeared to cause loss of central 

 vision, peripheral vision being intact, and came to the conclusion, partly 

 from this but mainly from the results of excitation of the occipital 

 cortex, 3 that the scheme given by Munk for the dog will not apply to 

 the monkey's brain, in which it is more than probable that central 

 vision is represented upon the mesial surface of the brain at the anterior 

 part of the visual area, which would be in the neighbourhood of the 

 anterior part of the calcarine fissure. And this appears certainly to be 

 the part where visual perceptions are most obviously located in man 

 (see p. 757). 



Electrical excitation of the visual area. As already remarked, it 

 was stated by Ferrier that electrical excitation of the angular gyrus 

 produces conjugate lateral movement of the eyes to the opposite 

 side ; with an upward deviation if the electrodes are placed on 

 the anterior limb of the gyrus, with a downward deviation if 

 placed on the posterior limb. 4 These experiments were subsequently 

 repeated by other observers, who for the most part failed to obtain 

 the same result. 5 I was myself never able to produce such move- 

 ments on excitation of the anterior limb of the angular gyrus, but 

 occasionally got the movement described by Ferrier when the elec- 

 trodes were placed on the posterior limb and on the upper end of the 

 middle temporal gyrus, which is continuous with the posterior limb. 

 In both these cases there may have been a spread of current to the 

 adjacent upper surface of the occipital lobe. If the electrodes are 

 applied directly to the upper surface of the occipital lobe, conjugate 

 deviation to the opposite side with downward direction of the visual 

 axes is extremely well marked, and it is also produced by excitation of 

 the upper part of the mesial surface of that lobe, and of the quadrate 

 lobule immediately in front of the internal parieto-occipital fissure. If, 

 on the other hand, the posterior extremity of the occipital lobe, its 

 lower or tentorial surface, and the posterior and lowermost part of its 

 mesial surface, be stimulated, the lateral deviation is combined with an 

 upward movement. In both cases the downward or upward movement 

 may be almost uncomplicated by lateral deviation ; this depends upon 

 the position of the electrodes. On the other hand, there is an inter- 

 mediate zone, narrow on the mesial surface of the lobe, and broader 

 upon the outer or convex surface, excitation of which is productive of 



i 



1 Vide infra. 



-Phil. Trans., London, 1888, B; Brain, London, vol. x. p. 370. 



3 Schafer, Brain, London, vol. xi. p. 1. 



4 "Functions of the Brain," 2nd edition, p. 243. 



5 Luciani and Tamburini obtained in dogs conjugate deviation to the opposite side, 

 both from the angular gyrus and from the convexity of the occipital lobe, but without any 

 upward or downward deflection, "Centri psico-sensori corticali," Reggie-Emilia, 1879. 



