IX. A further minute analysis by electric stimulation of the so-called motor region of the cortex cerebri in the monkey (macacus sinicus)

Abstract

In a former paper published in the ‘Philosophical Transactions‚’ B., vol. 178 (1887), pp. 153-167, a minute account was given by us of the effects produced by electrical stimulation of the so-called motor area in the cerebral cortex for the upper limb. In continuation of this work we have investigated in a similar manner the effects produced by stimulating the rest of the so-called motor region on the convexity of the cortex, with the exception of the lower parts of the ascending frontal and parietal convolutions. The parts examined in the present research comprise the posterior third of the frontal convolutions, the upper third of the ascending frontal convolution, the superior parietal lobule, and the posterior half of the ascending parietal convolution. The movements evoked by stimulating these areas were respectively those of the head and eyes to the opposite side, conjoined movement of both opposite limbs, movements of the lower limb, and movements of the upper limb. In order to avoid discrepancies in the arrangement of the cerebral sulei in different species, and so to obtain exact localisation of the effect produced, the same variety of Monkey was alone used, viz., Macacus sinicus . In all we have performed twentythree experiments, the animal in each case having been narcotised with ether and killed before recovery from the anæsthetic. PART I.—ANATOMY. In the paper above referred to, a description was given of the position in the Monkey of the fissure of Rolando, the præcentral sulcus, and a small fissure above this, called by Professor Schäfer “ x .” The excitable region investigated in the present paper is limited below, where it lies in front of the præcentral sulcus, by the horizontal level* of the lower end of that sulcus, the superior and anterior part being bounded above by the middle line, and in front by a sulcus which Professors Horsley and Schäfer provisionally named “ y ” (see fig. 1). Below, it is partially bounded by a sulcus which we have temporarily called “ w .” This sulcus, “ w .” runs horizon­tally from near the tip of the frontal lobe backwards to within a very short distance of the vertical limb of the præcentral sulcus, thus lying below and approximately parallel to the upper limb of the præcentral sulcus. The homology between this sulcus and corresponding parts of the human brain has not as yet been made out. The middle part of the region investigated in the present research is bounded above by the longitudinal fissure and below by the sulcus “ x .” The remaining part of this region, situated behind the fissure of Rolando, is bounded above by the longitudinal fissure, behind by the intraparietal sulcus, and below by the horizontal level of the lower end of that sulcus. The upper part of the ascending parietal convolution, or as it is also called the superior parietal lobule, is divided by a small sulcus which is usually vertical, and which has been designated “ z ” by Professors Horsley and Schäfer (see fig. 1).