CH. L.] THE VISUAL AREA 737 



A man or an animal who loses both eyes is blind, but in time 

 manages to find his way about. This is not the case when blindness 

 is produced by removal or disease of both occipital lobes ; here, the 

 sense of orientation is lost also, for the association of sensory 

 memories and motor impulses is then impossible. 



Kemoval of one occipital lobe will be followed by different results 

 in the two classes of animals just referred to. In those with pano- 

 ramic vision, the result will be blindness of the opposite eye, because 

 the decussation of the optic nerve is complete at the chiasma. 

 But in animals such as monkeys with stereoscopic vision (in 

 which the only decussating fibres are those which come from the 

 inner halves of the two retinae) removal of one occipital lobe, or 

 disease of that lobe in man, produces blindness of the same side of 

 each retina, or inability to see the opposite half of the visual field. 

 This is called hemianopsia ; the head and eyes are turned to one 

 side, namely, the side of injury (conjugate deviation to the side of the 

 injury). Such an operation does not destroy vision in the central 

 portion (macula lutea) of either retina, because each macula sends 

 impulses to both sides of the brain. Stimulation of one visual area 

 leads to a subjective sensation apparently coming from the same 

 halves of both retinae, and also excites the solitary cells of Meynert ; 

 this produces conjugate deviation of head and eyes towards the 

 opposite side to that stimulated. 



These solitary cells are so called because they are few and far 

 between ; they are large cells not at all unlike the Betz cells of the 

 motor cortex. Their axons, no doubt, pass in long association tracts 

 to the motor eye centre of the frontal region and to the corpora 

 quadrigemina. 



The optic radiations consist of (1) sensory fibres from the optic 

 tracts via the external geniculate bodies; (2) efferent fibres to the 

 centres for eye-movements ; and (3) association fibres, which are last 

 developed. The last named link one convolution to others, and the 

 two hemispheres together, and bring about association of ideas of 

 vision in both hemispheres, and with other sensations. A large 

 collection of such fibres runs horizontally through the grey matter. 

 This white stripe is often visible to the naked eye ; it is the anatomical 

 mark of the visuo-sensory cortex, and is called the line of Gennari. 

 The visuo-psychic region (fig. 451) has no line of G-ennari, but 

 possesses many small and medium-sized pyramidal cells in its outer 

 layers, which play the part of association units, where memory 

 pictures are stored and visual sensations correlated with those from 

 other sense-organs; the higher one ascends the animal scale, the 

 greater becomes the depth of this layer. 



The eye centre in the frontal lobe is separated, in the higher 

 apes and man, by inexcitable grey matter from the rest of the 



3 A 



