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see a page of Chinese print but it suggests nothing to us. A hemiopic disturbance of vision is one in which neither retina is affected in its totality, but in which, for example, the left portion of each retina is blind, so that the animal sees nothing situated in space towards its right Later observations have corroborated this hemiopic character of all the disturbances of sight from injury to a single hemisphere in the higher animals; and the question whether an animal's apparent blindness is sensorial or only psychic has, since Hunk's first publications, been the most urgent one to answer, in all observations relative to the function of sight
Goltz almost simultaneously with Terrier and Munk reported experiments which led him to deny that the visual function was essentially bound up with any one localized portion of the hemispheres. Other divergent results soon came in from many quarters, so that, without going into the history of the matter any more, I may report the existing state of the case as follows: *
In fishes, frogs, and lizards vision persists when the hemispheres are entirely removed. This is admitted for frogs and fishes even by Munk, who denies it for birds.
All of Munk's birds seemed totally blind (blind sense- rially) after removal of the hemispheres by his operation. The following of a candle by the head and winking at a threatened blow, which are ordinarily held to prove the retention of crude optical sensations by the lower centres in supposed hemisphereless pigeons, are by Munk ascribed to vestiges of the visual sphere of the cortex left behind by the imperfection of the operation. But Schrader, who operated after Munk and with every apparent guarantee of completeness, found that all his pigeons saw after two or three weeks had elapsed, and the inhibitions resulting from the wound had passed away. They invariably avoided even the slightest obstacles, flew very regularly towards certain perches, etc., differing toto code in these respects with certain simply blinded pigeons who were kept with
* The history up to 1886 may be found in A. Christian!: Zur Physlologie des Geblrnes (Berlin, 1865).
them for comparison. They did not pick up food strewn on the ground, however. Schrader found that they would do this if even a small part of the frontal region of the hemispheres was left, and ascribes their non-self-feeding when deprived of their occipital cerebrum not to a visual, but to a motor, defect, a sort of alimentary aphasia.*
In presence of such discord as that between Munk and his opponents one must carefully note how differently significant is less, from preservation, of a function after an operation on the brain. The loss of the function does not necessarily show that it is dependent on the part cut out; but its preservation does show that it is not dependent: and this is true though the loss should be observed ninety-nine times and the preservation only once in a hundred similar excisions. That birds and mammals can be blinded by cortical ablation is undoubted; the only question is, must they be so? Only then can the cortex be certainly called the ' seat of sight' The blindness may always be due to one of those remote effects of the wound on distant parts, inhibitions, extensions of inflammation,—interferences, in a word,— upon which Brown-Sequard and Goltz have rightly insisted, and the importance of which becomes more manifest every day. Such effects are transient; whereas the symptoms of deprivation (Aiisfallserscheinungen, as Goltz calls them) which come from the actual loss of the cut-out region must from the nature of the case be permanent Blindness in the pigeons, so far as it passes away, cannot possibly be charged to their seat of vision being lost, but only to some influence which temporarily depresses the activity of that seat. The same is true mutatis mutandis of all the other effects of operations, and as we pass to mammals we shall see still more the importance of the remark.
In rabbits loss of the entire cortex seems compatible with the preservation of enough sight to guide the poor animals' movements, and enable them to avoid obstacles. Christiani's observations and discussions seem conclusively
* PflUger's Archiv, vol. 44, p. 176. Munk (Berlin Academy Sitzsungsberichte, 1889, xxxi) returns to the charge, denying the extirpations of Schrader to be complete: '' Microscopic portions of the Sektphar* must remain."
to have established this, although Monk found that all his animals were made totally blind.''
In dogs also Munk found absolute stone-blindness after ablation of the occipital lobes. He went farther and mapped out determinate portions of the cortex thereupon, which he considered correlated with definite segments of the two retinas, so that destruction of given portions of the cortex produces blindness of the retinal centre, top, bottom, or right or left side, of the same or opposite eye. There seems little doubt that this definite correlation is mythological. Other observers, Hitzig, Goltz, Luciani, Loeb, Ex nor, etc., find, whatever part of the cortex may be ablated on one side, that there usually results a hemiopic disturbance of both eyes, slight and transient when the anterior lobes are the parts attacked, grave when an occipital lobe is the seat of injury, and lasting in proportion to the latter's extent According to Loeb, the defect is a dimness of vision (' hemiamblyopia') in which (however severe) the centres remain the best seeing portions of the retina, just as they are in normal dogs. The lateral or temporal part of each retina seems to be in exclusive connection with the cortex of its own side. The centre and nasal part of each seems, on the contrary, to be connected with the cortex of the opposite hemispheres. Loeb, who takes broader views than any one, conceives the hemiamblyopia as he conceives the motor disturbances, namely, as the expression of an increased inertia in the whole optical machinery, of which the result is to make the animal respond with greater effort to impressions coming from the half of space opposed to the side of the lesion. If a dog has right hemiamblyopia, say, and two pieces of meat are hung before him at once, he invariably turns first to the one on his left. But if the lesion be a slight one, shaking slightly the piece of meat on his right (this makes of it a stronger stimulus) makes him seize upon it first. If only one piece of meat be offered, he takes it, on whichever side it be.
When both occipital lobes are extensively destroyed total blindness may result Munk maps out his 'Seh
• A. Christian!- Zur Physiol, d. Qehirnes (Berlin, 1886),cuaps. 11. m. Iv. H. Munk : Berlin Akad. Stzgsb. 1884, my
Bphare' definitely, and says that blindness must result when the entire shaded part, marked A, A, in Figs. 12 and 13, is involved in the lesion. Discrepant reports of other observations he explains as due to incomplete
Fio. 12. Flo. u
The Doe's visual centre according to Hunk, the entire striated region, A. A, being the exclusive seat of vision, and the dark central circle, A', being correlated with the retinal centre of the opposite eye.
ablation. Luciani, Goltz, and Lannegrace, however, contend that they have made complete bilateral extirpations of Munk's Sehsphare more than once, and found a sort of crude indiscriminating sight of objects to return in a few weeks.'' The question whether a dog is blind or not is harder to solve than would at first appear; for simply blinded dogs, in places to which they are accustomed, show little of their loss and avoid all obstacles; whilst dogs whose occipital lobes are gone may run against things frequently and yet see notwithstanding. The best proof that they may see is that which Goltz's dogs furnished: they carefully avoided, as it seemed, strips of sunshine or paper on the floor, as if they were solid obstacles. This no really blind dog would do. Luciani tested his dogs when hungry (a condition which sharpens their attention) by strewing pieces of meat and pieces of cork before them. If they went straight at them, they saw; and if they chose the meat and left the cork, they saw discriminatingly. The quarrel is very acrimonious; indeed the subject of localization of functions in the brain seems to have a peculiar effect on the temper of those who cultivate it experimentally. The amount of preserved vision which Goltz and Luciani report seems hardly to be worth considering, on the one hand; and on the other, Munk admits in his penultimate paper that out of 85 dogs he only ' succeeded' 4 times in his operation of producing complete blindness by complete extirpation of his 'Sehsphare.' * The safe conclusion for us is that Luciani's diagram, Fig. 14, represents something like the
* Luciani uuil Seppili: Die Functions-Localization auf der Grosshirnrinde (Deutsch von Fraenkel), Leipzig, 1886, Dogs M, N, and S. Goltz in Ptiuger's Archiv, vol. 34, pp. 490-6; vol. 42, p. 454. Of. also Hunk: Berlin Akad. Stzgsb. 1886, vn, vm. pp. 113-121, and Loeb: PflOger's Archiv, vol. 39, p. 887.
Fio. u.- Distribution of the Visual Function in the Cortex, according to Luciani.
truth. The occipital lobes are far more important for vision than any other part of the cortex, so that their complete destruction makes the animal almost blind. As for the crude sensibility to light which may then remain, nothing exact is known either about its nature or its seal
In the monkey, doctors also disagree. The truth seems, however, to be that the occipital lobes in this animal also are the part connected most intimately with the visual function. The function would seem to go on when very small portions of them are left, for Ferrier found no 'appreciable impairment' of it after almost complete destruction of them on both sides. On the other hand, he found complete and permanent blindness to ensue when they and the angular gyri in addition were destroyed on both sides. Munk, as well as
* Berlin Akad. Sitzungsberichte, 1886, vn, vm, p. 134.