The cortex is subdivided into numerous Brodmann’s areas based on cytoarchitectural differences, which are convenient when referring to functions of different cortical locations. Deep to the cortex are myelinated fiber bundles, i.e., projection, association, and commissural, which interconnect adjacent gyri, distant gyri, the two hemispheres, and subcortical centers. Brodmann’s areas 3, 2,1 (on the postcentral gyrus) is the primary somatosensory cortex; area 17 (cuneus and lingual gyri) receives visual input; area 41 (transverse temporal gyrus) is for auditory sense. These areas receive information in precise somatotopic, retinotopic, and tonotopic order. Taste and vestibular information project to the postcentral gyrus just behind the somatosensory region of the head. Primary olfactory cortex is located medially and basally on the frontal and temporal lobes (area 34). The precentral gyrus contains the primary motor area (area 4) controlling volitional motor activity. It receives input from the premotor cortex (areas 6 & 8) and the somatosensory cortex. Areas 4,6,8 and 3,2,1 contribute fibers to the corticospinal tract. Area 8 also controls horizontal gaze. Areas 44, 45 on the left, comprise the motor speech area (of Broca). The anterior 2/3rds of the frontal lobes (prefrontal cortex) handles "initiative, thinking, emotions", etc. Injury to the prefrontal lobes does not produce major primary motor or sensory deficits but rather psychic disorders.
Each primary sensory area has an adjacent secondary area from which only secondary electrical potentials can be detected by sensory excitation. These associational areas of the parietal, temporal, and occipital lobes occupy the largest portions of those lobes. Areas 5 & 7 are secondary to somatosensory areas 3,2,1; areas 18 & 19 are secondary to visual area 17; areas 42 & 22 are secondary to auditory area 41; area 28 is secondary to olfactory area 34. Information projected to primary sensory cortices is integrated in secondary association areas for comparison with previously stored memories. This memory function is largely a property of the dominant hemisphere.
Parietal areas 39 and 40, i.e., the angular and supramarginal gyri, lie at the junction of the parietal, occipital, and temporal lobes and integrate tactile-kinesthetic, visual, and auditory association areas at the highest level. Areas 39 and 40 have connections with the thalamus, ipsilateral frontal, occipital, and temporal lobes, and corresponding contralateral cortical via the corpus callosum. Near areas 39 and 40, at the posterior margin of area 22, is Wernicke’s speech area; it is connected with Broca’s motor speech area by long associational fibers of the white matter, injury to Wernicke’s area results in receptive aphasia.
Most of the 300 million fibers of the corpus callosum and anterior commissure interconnect identical sites of the two hemispheres (some do not, e.g., the primary visual cortex) so that the hemispheres are in constant communication. Each hemisphere is better at performing certain functions than the other - this phenomenon is called dominance. The left hemisphere is usually better at language and math, i.e., the analytic, rational, and verbal; the right hemisphere is better for spatial relationships (e.g., recognition of faces), musicality, and creativity, i.e., the intuitive, synthetic, and non-verbal. For most people the left side is dominant for speech and language, spoken or written, though both sides can comprehend. Both sides can perceive tactile, visual, and auditory sensations but only the dominant hemisphere can respond verbally; the non-dominant side can demonstrate its comprehension by corresponding with gestures or emotion (e.g., blushing). About 90% of people are left side dominant for language; in about 80% of people the left hemisphere is dominant for motor skills so they are right handed.
Blood is supplied to the hemispheres by the paired internal carotid and vertebral arteries. Inside the cranium they form the circle of Willis and basilar artery (which supplies the brainstem). Internal carotids give rise to the anterior and middle cerebral arteries. Vertebral arteries fuse to form the basilar artery at the pontomedullary junction which divides at the junction of pons and midbrain to give rise to the posterior cerebral arteries. The lateral surface of most of the hemisphere is supplied by branches of the middle cerebral artery which lies in the lateral fissure. The medial surface is supplied by the anterior cerebral artery which follows the corpus callosum and supplies the medial frontal and parietal lobes. The posterior cerebral artery runs posteriorly and supplies the medial edge of the temporal and occipital lobes.