SECTION A. TUESDAY, August 2nd, 1892. PROFESSOR HITZIG, of Halle, Germany, IN THE CHAIR. Professor S. E. HENSCHEN, M.D., of Upsala, Sweden, read a paper On the Visual Path and the Visual Centre. Perception through our senses being the condition and basis of our psychical life, the question as to the situation and organisation of the cerebral areas of the senses ranks amongst the foremost and most important problems of psycho-physiology, and a certain and more penetrating knowledge of the nature of the psychical processes will not be reached without a solution of these problems. And the resolving of the higher psychical problems must be sought principally through researches in man, and also through combined clinical and pathological investigations. No sense is fitted to give such correct and detailed results of the organisation of its cerebral path and centre, as that of sight; because we can so exactly examine the disorders of its functions. In the following, I purpose giving a short sketch of the results of my clinico-anatomical investigations on the visual path and centre. The reasons for my opinions will be found in my Work Beitraege zur Pathologie des Gehirns." 1890 and 1892. The starting point is a clinical examination combined with a postmortem. The conditions for obtaining an exact result are that both the clinical and pathological examinations be thorough. But few cases will meet these requirements, although there are a great number of observations made on derangements of sight in diseases which are brought to post-mortem investigation. The negative cases without hemianopsia are often of special interest, as controlling the results indicated by the positive cases of hemianopsia. The Visual Path is divided into three portions: The frontal, middle and occipital. The frontal part stretches from the bulb to the external geniculate body, and is indicated by nature in a very manifest manner, as the optical nerve, the chiasma, and tractus. The fibres are arranged in separate bundles for the different quadrants of the retina, and the bundles for each lie distinct from each other. The middle portion. On entering into the external geniculate body fibres from the corresponding points of both retinas lie together, and, according to the opinion of Monakow, ramify, without immediate connection with the cells of this ganglion. Other bundles enter into the pulvinar and anterior tubercle of the corp. quadrigemina. The analysis of the clinical cases seems to determine the following points: That the external geniculate body is necessary to sight. Its destruction always provokes hemianopsia. There is no clinical proof that the destruction of the pulvinar or of the anterior tubercle will produce hemianopsia. Nor has the commonly accepted opinion, that a lesion of the posterior part of the internal capsule will produce hemianopsia, a clinical ground. The doctrine of a crossing of the fibres in the middle line of the corpora quadrigemina lacks the support of clinical facts. The occipital portion starts from the external geniculate body, forms a small bundle of fibres, a few millimètres thick, and lies in the optical radiation, as high as the second temporal gyrus, the second temporal sulcus and the calcarine fissure. Every lesion of this bundle will produce a defect in the visual field; lesions of other segments of the optical radiations do not produce any form of hemianopsia. A lesion of the cortex or medulla of the parietal lobe, and especially of the angular gyrus, will be followed by hemianopsia only if the above mentioned bundle be touched, directly or indirectly. Also the angular gyrus does not belong to the visual centre in man. In the optical bundle the fibres for the dorsal retinal quadrant have a dorsal, for the ventral, a more ventral, and for the macular fibres probably a more medial situation. The optical bundle terminates in the cortex of the calcarine fissure. A lesion of the lateral, dorsal, or ventral surface of the occipital lobe will produce a defect in the visual field only if it touches the cortex of the calcarine fissure, or the fibres emanating from it to the above-mentioned optical bundle. A lesion of the first occipital gyrus, the margo falcata, of the cuneus or lobulus linguatis does not produce any defect in the field of vision; but a lesion of the whole calcarine cortex will produce a complete hemianopsia. Also the visual centre is limited to this part. There is no exactly described clinical case which conflicts with these assertions. The negative cases further support them. Whether the calcarine cortex, in its whole length, belongs to the visual centre or not, cannot be decided by means of the present existing clinical facts. Organisation of the Visual centre. There exists a projection of the retina in the calcarine cortex, The dorsal lip corresponds with the dorsal retinal quadrant, the ventral with the ventral. The cortical macula lutea probably lies from three to four centimètres in front of the occipital tip, and the horizontal periphery nearer the tip; the halves of the macula lutea are often represented by an irregular variation in both hemispheres. In this circumscribed portion of the occipital cortex both retina halves are represented; and elements from corresponding points of the retinas probably lie side by side, which position facilitates binocular sight. To the calcarine cortex must also be referred colour-perception and not to the ventral cortex of the occipital lobe, as has been supposed. The above mentioned theory of the situation of the visual centre in man agrees with the results of: (1) Examination of brains after extirpation of the bulbs. (2) Examination of brains after destruction of the external geniculate body. |