into three series. His earliest interest was apparently in speech defect in brain disease, and by careful and detailed study of numerous cases he was able to associate such defect in most cases with disease in the left cerebral hemisphere. His papers with these detailed facts and conclusions were published chiefly in the 'London Hospital Reports' in and albout 1864. Two years previously Broca had definitoly associated loss of speech with disease of the posterior part of the third left frontal convolution. These investigations were unknown to Jackson at the date of his early research, and on learning of them he generously acknowledged that his independent conclusions had 'on every point of importance been anticipated by M. Broca.' The exceptions noted by Jackson were subsequently found to be explained in most instances by the observation that in left-handed persons the speech centre was as a rule situated in the right hemisphere.
The second series of Dr. Jackson's investigations was concerned with the occurrence of local epileptic discharges. These are now known as instances of Jacksonian epilepsy, although Jackson did not himself use that term. He always acknowledged Bravais's earlier recognition of this form of convulsion (1824), and the observation of 'epileptic hemiplegia' — the temporary paralysis following such convulsions — by Dr Robert Bentley Todd [q. v.]. But it was by the observation of a large number of such cases of convulsions starting locally, by careful examination of the subsequent paralysis or weakness, and the correlation of these with the actual position of the lesion in the brain giving rise to the phenomena, that Jackson was able, in 1870, to indicate certain regions of the brain as definitely related to certain limb movements, as well as to confirm incidentally the earlier work by Broca on the speech centre. Fritsch and Hitzig in Germany, and Ferrier in England, soon supplied experimental corroboration.
Jackson's third series of investigations had reference to the hierarchy of the nervous system, and although it may seem more theoretical and suggestive than practical, yet his hypotheses were constantly fortified and illustrated from clinical observation and the study of actual disease. He conceived the nervous system to consist of a series of levels — a lower, a middle, and a higher. In the lowest level, movements are represented in their simplest and least complex form; these centres are situated in the medullary and spinal structures. The middle level consists of the so-called motor area of the cortex, and the highest motor levels are found in the prefrontal area. Jackson did not attempt to formulate definitely the application of this theory of levels to sensory structures. His conception of the nervous system, as an evolution of the complex out of the simple, renders intelligible the theory of nervous disease as a process of dissolution — a term borrowed from Herbert Spencer. The highest and most lately developed functions are those to go first in the process of disease. The removal of the inhibition of the highest centres results in the uncontrolled action of the lower, and we thus have the explanation of such widely different conditions as post-hemiplegic rigidity and the illusions of the insane. Negative or destructive lesions do not produce positive symptoms; these are the outcome of the action of normal structures acting without the control or restraint of the more highly developed structures or structures of the higher level. The last subject at which he worked was the form of epilepsy which has been designated 'uncinate,' from the fact, which he was the first to point out, that its symptoms were associated with a lesion in the uncinate gyrus of the temporo-sphenoidal lobe. His first case of this disorder was published in 1866, and he returned to the subject in several later contributions to medical literature.
Jackson's researches depended on an immense amount of detailed observation. Thousands of cases were carefully diagnosed, and their symptoms and signs noted in the greatest detail. His work combines attention to the minutest details with a power of the widest generalisation. As a clinical assistant at Moorfields Eye Hospital Jackson was one of the first physicians to use the ophthalmoscope in this country, and he employed it habitually and diligently in his observations on disease. He was the first to point out that well-marked optic neuritis may co-exist with perfect vision.
Jackson, whose personal character was notable for its simplicity and consideration for others, died at 3 Manchester Square on 7 Oct. 1911, and was buried at Highgate. He married in 1865 his cousin, Elizabeth Dade Jackson; she died in 1876, leaving no issue.
Jackson's writings have not been collected. They are scattered through various periodicals. The 'London Hospital Reports,' 1864-1869, contain some of his earliest
