James Taylor

“ 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. ”