Brown and Schaefer, found no disturbance of sight from destroying the angular gyri alone, although Ferrier found blindness to ensue. This blindness was probably due to inhibitions exerted in distans, or to cutting of the white optical fibres passing under the angular gyri on their way to the occipital lobes. Brown and Schaefer got complete and permanent blindness in one monkey from total destruction of both occipital lobes. Luciani and Seppili, performing this operation on two monkeys, found that the animals were only mentally, not sensorially, blind. After some weeks they saw their food, but could not distinguish by sight between figs and pieces of cork. Luciani and Seppili seem, however, not to have extirpated the entire lobes. When one lobe only is injured the affection of sight is hemiopic in monkeys: in this all observers agree. On the whole, then, Munk's original location of vision in the occipital lobes is confirmed by the later evidence.*

In man we have more exact results, since we are not driven to interpret the vision from the outward conduct. On the other hand, however, we cannot vivisect, but must wait for pathological lesions to turn up. The pathologists who have discussed these (the literature is tedious ad libitum) conclude that the occipital lobes are the indispensable part for vision in man. Hemiopic disturbance in both eyes comes from lesion of either one of them, and total blindness, sensorial as well as psychic, from destruction of both.

Hemiopia may also result from lesion in other parts, especially the neighboring angular and supra-marginal gyri, and it may accompany extensive injury in the motor region of the cortex. In these cases it seems probable that it is due to an actio in distans, probably to the interruption of

* H. Munk: Functionen der Grosshirnrinde (Berlin, 1881), pp. 36-40. Ferrier Functions, etc., 2d ed., chap. Ix, pt. 1. Brown and Schaefer: Philos. Transactions, vol. 179, p. 321. Luciani u. Seppili, op. cit. pp. 131-138. Lannegrace found traces of sight with both occipital lobes destroyed, and in one monkey even when angular gyri and occipital lobes were destroyed altogether. His paper is in the Archives de Médecine Expérimentale for January and March, 1889. I only know it from the abstract in the Neurologisches Centralblatt, 1889, pp. 108–420. The reporter doubts the evidence of vision in the monkey. It appears to have consisted in avoiding obstacles and in emotional disturbance in the presence of men,

fibres proceeding from the occipital lobe. There seem to be a few cases on record where there was injury to the occipital lobes without visual defect. Ferrier has collected as many as possible to prove his localization in the angular gyrus. A strict application of logical principles would make one of these cases outweigh one hundred contrary ones. And yet, remembering how imperfect observations may be, and how individual brains may vary, it would certainly be rash for their sake to throw away the enormous amount of positive evidence for the occipital lobes. Individual variability is always a possible explanation of an anomalous case. There is no more prominent anatomical fact than that of the 'decussation of the pyramids,' nor any more usual pathological fact than its consequence, that left-handed hemorrhages into the motor region produce right-handed paralyses. And yet the decussation is variable in amount, and seems sometimes to be absent altogether.t If, in such a case as this last, the left brain were to become the seat of apoplexy, the left and not the right half of the body would be the one to suffer paralysis.

The schema on the opposite page, copied from Dr. Seguin, expresses, on the whole, the probable truth about the regions concerned in vision. Not the entire occipital lobes, but the so-called cunei, and the first convolutions, are the cortical parts most intimately concerned. Nothnagel agrees with Seguin in this limitation of the essential tracts.‡

A most interesting effect of cortical disorder is mental blindness. This consists not so much in insensibility to optical impressions, as in inability to understand them. Psychologically it is interpretable as loss of associations between optical sensations and what they signify; and any interruption of the paths between the optic centres and the centres for other ideas ought to bring it about. Thus,

Localization of Cerebral Disease (1878), pp. 117-8.

For cases see Flechsig: Die Leitungsbahnen in Gehirn u. Rückenmark (Leipzig, 1876), pp. 112, 272; Exner's Untersuchungen, etc., p. 83; Ferrier s Localization, etc., p. 11; François-Franck's Cerveau Moteur, p. 63, note.

E. C. Seguin : Hemianopsia of Cerebral Origin, in Journal of Nervous and Mental Disease, vol. xIII. p. 30. Nothnagel und Naunyn: Ueber die Localization der Gehirnkrankheiten (Wiesbaden, 1887), p. 10.

printed letters of the alphabet, or words, signify certain sounds and certain articulatory movements. If the connection between the articulating or auditory centres, on the one hand, and the visual centres on the other, be ruptured,

FIG. 15.-Scheme of the mechanism of vision, after Seguin. The cuneus convolution (Cu) of the right occipital lobe is supposed to be injured, and all the parts which lead to it are darkly shaded to show that they fail to exert their function. F. O. are the intra-hemispheric optical fibres. P. O. C. is the region of the lower optic centres (corpora geniculata and quadrigemina). T. O. D. is the right optic tract; C, the chiasma; F. L. D. are the fibres going to the lateral or temporal half T of the right retina; and F. C. S. are those going to the central or nasal half of the left retina. O. D. is the right, and O. S. the left eyeball. The rightward half of each is therefore blind: in other words, the right nasal field, R. N. F., and the left temporal field, LT. F., have become invisible to the subject with the lesion at Cu.

we ought a priori to expect that the sight of words would fail to awaken the idea of their sound, or the movement for pronouncing them. We ought, in short, to have alexia, or inability to read: and this is just what we do have in many

cases of extensive injury about the fronto-temporal regions, as a complication of aphasic disease. Nothnagel suggests that whilst the cuneus is the seat of optical sensations, the other parts of the occipital lobe may be the field of optical memories and ideas, from the loss of which mental blindness should ensue. In fact, all the medical authors speak of mental blindness as if it must consist in the loss of visual images from the memory. It seems to me, however, that this is a psychological misapprehension. A man whose power of visual imagination has decayed (no unusual phenomenon in its lighter grades) is not mentally blind in the least, for he recognizes perfectly all that he sees. On the other hand, he may be mentally blind, with his optical imagination well preserved; as in the interesting case published by Wilbrand in 1887.* In the still more interesting case of mental blindness recently published by Lissauer,† though the patient made the most ludicrous mistakes, calling for instance a clothes-brush a pair of spectacles, an umbrella a plant with flowers, an apple a portrait of a lady, etc. etc., he seemed, according to the reporter, to have his mental images fairly well preserved. It is in fact the momentary loss of our non-optical images which makes us mentally blind, just as it is that of our non-auditory images which makes us mentally deaf. I am mentally deaf if, hearing a bell, I can't recall how it looks; and mentally blind if, seeing it, I can't recall its sound or its name. As a matter of fact, I should have to be not merely mentally blind, but stone-blind, if all my visual images were lost. For although I am blind to the right half of the field of view if my left occipital region is injured, and to the left half if my right region is injured, such hemianopsia does not deprive me of visual images, experience seeming to show that the unaffected hemisphere is always sufficient for production of these. To abolish them entirely I should have to be deprived of both occipital lobes, and that would deprive me not only of my inward images of sight, but of my

* Die Seelenblindheit, etc., p. 51 ff. The mental blindness was in this woman's case moderate in degree.

+ Archiv f. Psychiatrie, vol. 21, p. 222.

sight altogether.* Recent pathological annals seem to offer a few such cases. † Meanwhile there are a number of cases of mental blindness, especially for written language, coupled with hemianopsia, usually of the rightward field of view. These are all explicable by the breaking down, through disease, of the connecting tracts between the occipital lobes and other parts of the brain, especially those which go to the centres for speech in the frontal and temporal regions of the left hemisphere. They are to be classed among disturbances of conduction or of association; and nowhere can I find any fact which should force us to believe that optical images need be lost in mental blindness, or that the cerebral centres for such images are locally distinct from those for direct sensations from the eyes. §

Where an object fails to be recognized by sight, it often happens that the patient will recognize and name it as soon as he touches it with his hand. This shows in an interest

*Nothnagel (loc. cit. p. 22) says: "Dies trifft aber nicht zu." He gives, however, no case in support of his opinion that double-sided cortical lesion may make one stone-blind and yet not destroy one's visual images; so that I do not know whether it is an observation of fact or an a priori assumption.

In a case published by C. S. Freund: Archiv f. Psychiatrie, vol. xx, the occipital lobes were injured, but their cortex was not destroyed, on both sides. There was still vision. Cf. pp. 291-5.

I say 'need,' for I do not of course deny the possible coexistence of the two symptoms. Many a brain-lesion might block optical associations and at the same time impair optical imagination, without entirely stopping vision. Such a case seems to have been the remarkable one from Charcot which I shall give rather fully in the chapter on Imagination.

Freund (in the article cited above Ueber optische Aphasie und Seelenblindheit ') and Bruns (Ein Fall von Alexie,' etc., in the Neurologisches Centralblatt for 1888, pp. 581, 509) explain their cases by brokendown conduction. Wilbrand, whose painstaking monograph on mental blindness was referred to a moment ago, gives none but a priori reasons for his belief that the optical ‘Erinnerungsfeld' must be locally distinct from the Wahrnehmungsfeld (cf. pp. 84, 93). The a priori reasons are really the other way. Mauthner ('Gehirn u. Auge' (1881), p. 487 ff.) tries to show that the mental blindness' of Munk's dogs and apes after occipital mutilation was not such, but real dimness of sight. The best case of mental blindness yet reported is that by Lissauer, as above. The reader will also do well to read Bernard: De l'Aphasie (1885) chap. v; Ballet: Le Langage Intérieur (1886), chap. VIII; and Jas. Ross's little book on Aphasia (1887),