698 THE BRAIN. 



conclusion, though there is much discordance among the various observers, 

 both as to the particular results and especially as to their interpretation. 

 One broad fact comes out in all the observations, namely, that the removal 

 of or injury to the hind region of the cortex always produces some disturb- 

 ance 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. 149 and 150) 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 impair- 

 ment might be noticed long afterward, 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. 148) has led to hemiopia, failure in the left (or right) visual field, 

 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 ob- 

 served in which the transient blindness due to removal of the occipital lobes 

 has been succeeded by permanent hemiopia upon the subsequent removal of 

 the angular gyrus. Indeed, the general, but not uniform, tendency of the 

 many experiments which have been made is to connect, in the monkey, both 

 the occipital lobe and the angular gyrus with vision. 



In the dog, removal of portions of the occipital cortex have also led to 

 partial and transient blindness, or, according to some, to permanent blind- 

 ness ; but the difficulties of judging the visual condition of a dog are very 

 considerable, and his vision is so different from that of a man, so much less 

 binocular, for instance, than his, that it would not be profitable to relate at 

 length the results obtained in the dog or to discuss the conclusions which 

 have been derived from them. We will only say that some observers have 

 been led to think that the lateral part of the retina is connected with the 

 lateral part of the visual occipital area, the front part with the front part, 

 and so on, the retina being, as it were, projected on to the occipital cortex ; 

 but the facts are not clear enough to make it worth while to dwell upon 

 them here. 



In man, clinical histories so far conform to the results of experiments on 

 the monkey as to associate the occipital cortex, and more particularly the 

 cuneus (see Figs. 152 and 153), with vision. They have, however, raised a 

 point on which we have not yet touched. In the experiments on the mon- 

 key quoted above, the result (putting aside transient effects due probably to 

 " shock ") of interference with one side of the brain was hemiopia ; and this 

 is what we might expect from the anatomical relations ; the optic tract goes 

 straight to the tegmental masses of its own side, and the optic radiation 

 passes from those masses to the occipital cortex of the same side ; there is 

 no decussation, save of the fibres of the optic nerve, as they pass into the 



