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External Morphology of the

Central Nervous System

Figure 2-3 on facing page) follow their respective roots. The posterior spinal artery is found medial to the entering posterior rootlets (and the dorsolateral sulcus), while the anterior spinal artery is in the anterior median fissure (see also Figure 2-2, facing page).

2-1 Posterior (upper) and anterior (lower) views showing the general features of the spinal cord as seen at levels C2~C5. The dura and arachnoid are reflected, and the pia is intimately adherent to the spinal cord and rootlets. Posterior and anterior spinal medullary arteries (see

Figure 2-3 on facing page) follow their respective roots. The posterior spinal artery is found medial to the entering posterior rootlets (and the dorsolateral sulcus), while the anterior spinal artery is in the anterior median fissure (see also Figure 2-2, facing page).

2-2 Posterior (upper) and anterior (lower) views showing details of the spinal cord as seen in the C7 segment. The posterior (dorsal) root ganglion is partially covered by dura and connective tissue.

2-3 Semidiagrammatic representation showing the origin and general location of principal arteries supplying the spinal cord. The anterior and posterior radicular arteries arise at every spinal level and serve their respective roots and ganglion. The anterior and posterior spinal medullary arteries (also called medullary feeder arteries or segmental medullary arteries) arise at intermittent levels and serve to augment the blood supply to the spinal cord. The artery of Adamkiewicz is an unusually large spinal medullary artery arising usually on the left in low thoracic or upper lumbar levels (T9—Lj). The arterial vasocorona is a diffuse anastomotic plexus covering the cord surface.

2-4 Overall posterior (A,B) and sagittal MRI (C, T2-weighted) views of the lower thoracic, lumbar, sacral, and coccygeal spinal cord segments and the cauda equina. The dura and arachnoid are retracted in A and B. The cauda equina is shown in situ in A, and in B the nerve roots of the cauda equina have been spread laterally to expose the conus medullaris and filum terminale internum. This latter structure is also called the pial part of the filum terminale. See Figures 5-1 and 5-2 on pages 84—87 for cross-sectional views of the cauda equina.

In the sagittal MRI (C), the lower portions of the cord, the filum terminale internum, and cauda equina are clearly seen. In addition, the intervertebral discs and the bodies of the vertebrae are clear. The lumbar cistern is an enlarged part of the subarachnoid space caudal to the end of the spinal cord. This space contains the anterior and posterior roots from the lower part of the spinal cord that collectively form the cauda equina. The filum terminale internum also descends from the conus medullaris through the lumbar cistern to attach to the inner surface of the dural sac. The dural sac ends at about the level of the S2 vertebra and is attached to the coccyx by the filum terminale externum (also see Fig. 2-47 on page 47). A lumbar puncture is made by inserting a large gauge needle (18-22 gauge) between the L3 and L4 vertebra or L4 and L5 vertebra and retrieving a sample of cerebrospinal fluid from the lumbar cistern. This sample may be used for a number of diagnostic procedures.

2-5 Lateral (A) and medial (B) views of the cerebral hemisphere showing the landmarks used to divide the cortex into its main lobes.

On the lateral aspect, the central sulcus (of Rolando) separates frontal and parietal lobes. The lateral sulcus (of Sylvius) forms the border between frontal and temporal lobes. The occipital lobe is located caudal to an arbitrary line drawn between the terminus of the parieto-occipital sulcus and the preoccipital notch. A horizontal line drawn from approximately the upper two-thirds of the lateral fissure to the rostral edge of the occipital lobe represents the border between parietal and temporal lobes. The insular cortex (see also Figs. 2-46 on page 45 and 3-1 on page 56) is located internal to the lateral sulcus. This part of the cortex is made up of long and short gyri that are separated from each other by the central sulcus of the insula. The insula, as a whole, is separated from the adjacent portions of the frontal, parietal, and temporal opercula by the circular sulcus.

On the medial aspect, the cingulate sulcus separates medial portions of frontal and parietal lobes from the limbic lobe. An imaginary continuation of the central sulcus intersects with the cingulate sulcus and forms the border between frontal and parietal lobes. The parieto-occipital sulcus and an arbitrary continuation of this line to the preoc-cipital notch separate the parietal, limbic, and temporal lobes from the occipital lobe.

2-6 Lateral (A) and medial (B) views of the cerebral hemisphere showing the more commonly described Brodmann areas. In general, area 4 comprises the primary somatomotor cortex, areas 3,1, and 2 the primary somatosensory cortex, and area 17 the primary visual cortex. Area 41 is the primary auditory cortex, and the portion of area 6 in the caudal part of the middle frontal gyrus is generally recognized as the frontal eye field.

The inferior frontal gyrus has three portions: a pars opercularis, pars triangularis, and a pars orbitalis. A lesion that is located primarily in areas 44 and 45 (shaded) will give rise to what is called a Broca aphasia, also called expressive or nonfluent aphasia.

The inferior parietal lobule consists of supramarginal (area 40) and angular (area 39) gyri. Lesions in this general area of the cortex (shaded), and sometimes extending into area 22, will give rise to what is known as Wernicke aphasia, also sometimes called receptive or fluent aphasia.

The Brain: Lobes 15

Precentral gyrus (primary somatomotor cortex)

Hip area

Posrcentral gyrus (primary somatosensory cortex)

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