The average adult brain weighs about 1400 grams (3 pounds), or approximately 2 percent of the total body weight.
The brain weighs about the same as the liver. But, information is what it's designed to digest and resynthesize, not meals. Keep referring to this chapter as the rest of the book unfolds.
The simplified version of the brain in figure 3 shows major landmarks on the outer surface as viewed from the left side. At left, the prefrontal cortex occupies most of the convex portion of the frontal lobe. Just behind it is the primary motor cortex, the central fissure, and the primary somatosensory cortex. Within the parietal lobe, the superior parietal lobule is that small uppermost portion. The intra-parietal sulcus is the boundary separating it from the larger inferior parietal lobule beneath. The occipital lobe is at the far right. The long temporal lobe extends from it.
Below the cerebrum lie the cerebellum and the brainstem. The pons appears as a small bulge in the uppermost portion of the brainstem. Below it, the oblique line points to the medulla, from which the spinal cord descends. The midbrain is hidden beneath the temporal lobe. The letters A and H refer to the much deeper locations of the amygdala and hippocampus in the medial temporal lobe.
In figure 4 we look up at the medial surface of the right hemisphere from below. The cerebellum has been removed from above and behind the brainstem. Now, the whole (shaded) undersurface of the inferotemporal and occipital region can be seen.
Two frontal lobe gyri are identified. The gyrus rectus lies along its lower medial surface. The orbital gyri are shaded to indicate that they lie along its undersur-face. In back, the precuneus, cuneus, and retrosplenial region are identified, because they are three of the most metabolically active regions of the resting brain.
Note that major parts of the limbic system lie disposed in the form of a large oval around the brain's inner surface. By convention, the limbic system includes the cingulate gyrus and parahippocampal gyrus, the hippocampus and amygdala, and the hypothalamus. The mammillary body and arcuate nucleus are parts of the hypothalamus.
Longer dashed lines suggest the position of the right thalamus. In front, the large dotted stippling emphasizes how extensively the medial dorsal nucleus of the thalamus interacts, reciprocally, with the whole prefrontal cortex. In back, the faint stippling emphasizes that the pulvinar interacts with the cuneus. The
diagonal lines on the long cingulate gyrus suggest its extensive reciprocal interactions with the anterior thalamic nucleus.
The fusiform gyrus in the inferior temporo-occipital cortex includes a major color-sensitive region. This will become important to the discussion in part IX. In the rest of part III, we take up the functional anatomy of the landmark regions shown in these figures.
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