Motor Area (Figs. 74, 75, 76 and 77).—The emissive motor area is situated in the anterior wall of the central sulcus, in the posterior one-half of the gyrus centralis anterior and in that part of the paracentral lobule immediately continuous with it. This is the center for ordinary voluntary motion on the opposite side of the body. .Axones from this area descend to the nuclei of all motor nerves. In lateral sclerosis there is degeneration and disappearance of the giant pyramidal cells limited to this motor area (Campbell). It is divided into four segments: the head and neck, the arm, the trunk, and the leg, named from below upward. The first extends to the inferior knee of the sulcus centralis (Rolandi), though eye movements appear to be represented in the posterior end of the middle frontal gyrus; the arm area comprises the region between the genu inferius and the genu superius, the thumb, fingers, wrist, forearm, arm and shoulder movements being represented in this ascending order; just above the shoulder area, at the genu superius, is the trunk area; and above that, in the anterior central gyrus and in the paracentral lobule, in front of the sulcus centralis, is the center for leg movements. The representation in the leg center is inverted, the ascending order being hip, thigh, leg, ankle, toes and great toe.
Motor points were first positively located by G. Fritsch and E. Hitzig in the dog's brain (187o). They located centers for (I) the neck muscles, (2) the foreleg extensors and adductors, (3) the flexors and rotators of the foreleg, and (4) the hind-leg muscles—all in the sigmoid gyrus; and a center (5) for the face muscles in the coronal gyrus. I have demonstrated a trunk center in the sigmoid gyrus, between the foreleg and hind-leg centers, stimulation of which causes flexion of the spine.
The psychic motor areas, or areas for educated movements are located just anterior to the above motor areas, in the anterior central gyrus and in the contiguous ends of the superior, middle and inferior frontal gyri (Figs. 74 and 75). These areas are believed to send their axones to the emissive motor centers in the cortex. The psychic motor center for the lower extremity is probably located just in front of its emissive motor center in the anterior central and superior frontal gyri. In the posterior end of the middle frontal gyrus is the psychic motor center for the arm, the writing center of Gordinier; and in the inferior frontal gyrus the center for the organs of voice and speech, hence the motor speech center. In right-handed people these centers are developed only in the left cerebral hemisphere.
The writing center was definitely located by Gordinier in 1899 (Amer. Jour. Med. Sciences). Paul Broca located the center of articulate speech in 1861. Pierre Marie contends that the center of articulation is not in the inferior frontal gyrus but is more deeply situated, possibly in the lentiform nucleus.
Common Sensory Area (Figs. 74 and 75).—According to Dr. Alfred W. Campbell the receptive area of common sensation is limited to the posterior wall of the sulcus centralis, including the anterior one-half of the posterior central gyrus and that part of the paracentral lobule which is continuous with it. This area undergoes exclusive Nissl degeneration in locomotor ataxia (Campbell). It is probably divided into segments similar to those of the motor area (Spiller).
Psychic Sensory Area (Figs. 74, 75, 76 and 77).—A large por tion of the remainder of the parietal cortex probably constitutes a number of centers for the interpretation of common sensory impulses, hence the term, psychic sensory area. Impressions of the muscular sense are believed to be interpreted in the supra marginal gyrus and the center of stereognosis is said to be located in the superior parietal lobule and prxcuneus. Perhaps other parts interpret tactile and temperature impulses. The whole receptive and psychic area of common sensation has been called the sonuesthetic area (Barker), though the application of this term might better be limited to the receptive area.
Acustic Center (Figs. 74 and 76).—The receptive acustic cen ter is located in the transverse temporal gyri and in that part of the superior temporal gyms which is continuous with them. In the adjacent part of the superior and middle temporal gyri, in the left hemisphere, is the psychic acustic center.
Optic Center (Figs. 75 and 77).—In the cuneus and lingual gyrus is located the receptive optic center for the temporal half of the same retina and the nasal half of the opposite one; perhaps, also, for the macula lutea of both sides. The re mainder of the occipital lobe and, according to Mills and others, the angular gyrus, also, form the psychic optic center. This latter center is probably unilateral and developed only in the left hemisphere of right-handed people.
The visual and acustic centers were located by a host of inves tigators inspired by the work of Fritsch and Hitzig. Among them were David Ferrier, Monck, and others.
Olfactory and Gustatory Centers (Figs. 75 and 77).—The uncus hippocampi and nucleus amygdalw form the chief cortical center of smell; in close association with them, there are the hippo campus, the dentate fascia and the callosal gyri. The gusta tory center formerly was thought to be in the anterior end of the fusiform gyms. Paul Flechsig in his recent studies of the human brain locates taste in the gyrus cinguli contiguous to the splenium of the corpus callosum (Fig. 75). The olfactory, auditory, visual, common sensory and motor areas are all dis tinguished' by a definite characteristic histological structure peculiar to each region (Campbell). Medullation of the fibers in these cortical areas occurs at different times; and, according to Flechsig, in the following order: olfactory, tactile and muscular sense, visual, auditory, and gustatory.