13. Stimulation of the convolution directly in front of the outer end of the crucial sulcus causes dilatation of the eyelids and pupil while the eyes and head are turned toward the opposite side.
14. Stimulation of the convolution behind the crucial fissure causes contraction of the muscles of the perineum.
15. Stimulation of the convolution in front of the crucial fissure, on its anterior and sloping portion, causes movements of the pharynx and larynx (swallowing).
16. Stimulation of motor areas of the cortex, by scraping, irritation, or disease tends to produce spasmodic contractions of certain groups of muscles (Jacksonian Epilepsy). Strong stimulation may cause general epileptiform spasms, which are at first tonic, then clonic. One such seizure strongly predisposes to a second. If, during an attack, the cortical centres presiding over a special group of muscles were sliced off, such muscles relaxed, though the general spasms in the other muscles continued.
Localizations of Spinal Lesions.
Being at once a conductor between the brain and nerves, and a reflex nerve centre, we must consider both rôles in seeking to locate lesions from symptoms. In passing from the nerves to and from the sensorium both sensory and motor currents cross so that one side of the brain presides over the other side of trunk and limbs. This crossing of the motor fibres takes place in the medulla oblongata, while that of the sensory fibres occurs in the spinal cord close in front of the nerve from which they have entered.
Cross-Section of one lateral half of the spinal cord therefore causes motor paralysis and rise of temperature of the whole of that side of the body posterior to the lesion, while it induces sensory paralysis and cooling on the opposite side of the body up to the same point. A very limited sensory paralysis on the same side occurs corresponding to the few sensory fibres passing outward obliquely through the portion injured by the cross-section.
A vertical section of the cord separating the one lateral half from the other does not necessarily affect the motor currents, while it produces a limited anæsthesia on each side in the area of distribution of the nerves, the sensory fibres of which crossed in the seat of the lesion.
Transverse section of the superior columns causes hyperæsthesia and lack of coördination.
Transverse section of inferior columns, or of the inferior horn of gray matter, if close behind the medulla, causes no motor paralysis, but if farther back induces motor paralysis on the same side of the body.