CHAP. IL] THE BRAIN. 1081 



current, playing upon the beginnings of the pyramidal fibres, is 

 able to inaugurate simple motor impulses or something like them, 

 whereas elsewhere the molecular changes induced by the current 

 are too confused to reach their normal expression. There can 

 be no doubt of course that molecular changes in this or that 

 part of the brain, set going by processes other than actual visual 

 impulses along the optic nerves, may give rise to visual sensations ; 

 and as we shall see in dealing with the senses the subject of 

 such ' subjective ' sensations is unable to distinguish them from 

 sensations of ' objective ' origin ; but it is at least unlikely that 

 the coarse disturbances started by a tetanizing current should 

 take such a definite form. Moreover the view in question is 

 disproved by the experimental result that the same movements 

 are brought about when the cortex is pared away and the 

 electrodes are applied to the subjacent white matter. This 

 result suggests the existence of efferent tracts or bundles of a 

 special kind, differing from those of the pyramidal kind, though 

 like them making connections with the ocular and other muscles ; 

 we have, however, as yet no other evidence of such tracts ex- 

 isting. 



672. The results of removal of the cortex also support the 

 same general conclusion, though there is much discordance among 

 the various observers both as to the particular results and es- 

 pecially as to their interpretation. One broad fact comes out in 

 all the observations, namely, that removal of or injury to the 

 hind region of the cortex always produces some disturbance of 

 vision, and produces disturbance of vision more surely and to a 

 greater extent than does injury to or removal of any other region 

 of the cortex ; but beyond this broad fact there is much dispute, 

 and we must be content here with a very brief statement. 



In the monkey some observers have found that removal of the 

 occipital lobe on one side, the region marked "vision" in Figs. 126, 

 127, caused hemiopia, the effect on the visual fields being a crossed 

 one ; when the right lobe was removed there was blindness in the 

 left visual fields, that is in the right halves of the retinas of both 

 eyes ; in other words the visual impulses passing along the right 

 optic tract failed to produce their usual effect, so that the animal 

 disregarded objects on its left hand side. We may remark that 

 the decussation of the optic nerves in the monkey is very similar 

 to that in man. When both occipital lobes were removed, total 

 blindness resulted. But, and this is most important, not only 

 was the hemiopia, caused by the removal of one lobe, transient, 

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

 after the total removal of both lobes, though some impairment 

 might be noticed long afterwards, so long in fact as the animal 

 was kept alive. 



In the hands of other observers destruction of the angular 

 gyrus of one side (Fig. 125) has led to hemiopia, failure in the left 



