Anatomy of the central nervous system
The central nervous system [systema nervorum centrale] or organ is an aggregation of nuclei, fasciculi and commissures - a large axis of grey and white substance situated in the dorsal mid-line of the body - and the bundles of fibres connecting it with the tissues of other systems and with the peripheral ganglia are of necessity correspondingly large. So numerous are the axones connecting it and so intimately are its neurones associated that a disturbance affecting any one part of the system may extend to influence all other parts. The enlarged cephalic extremity of this central axis, the brain or encephalon, is a special ag- gregation of nuclei and masses of grey substance, many of which are much larger than any found in the periphery.
In the study of the central nervous system its enveloping membranes or meninges are met with first, and logically should be considered first, but since a comprehensive description of these membranes involves a foreknowledge of the various structures with which they are related, it is more expedient to consider them after making a closer study of the entire system they envelop.
For convenience of study, the central nervous system is separated into the gross divisions, spinal cord and brain (encephalon). Each of these divisions will be subdivided and considered with especial reference to its anatomical and functional relations to the other divisions and the inter- relations of its component parts.
The olfactory nerve-fibers are the central processes of the bipolar olfactory nerve cell-bodies situated in the olfactory region of the nasal mucous membrane. In man, the olfactory region comprises the epithelium upon the superior third of the nasal septum and that upon practically the whole of the superior nasal concha.
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The facial or seventh nerve is purely motor. It is accompanied a short distance by a bundle usually called its sensory root or the intermediate nerve.
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Customarily, the cranial nerves are described as comprising twelve pairs and each is referred to by number.
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The trigeminus is the largest of the cranial nerves with the exception of the optic. It is usually described as the fifth cranial nerve and as possessing both a sensory and a motor root.
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The spinal nerves are arranged in pairs, the nerves of each pair being symmetrical in their attachment to either side of their respective segment of the spinal cord, and, in general, symmetrical in their course and distribution.
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The fibers of each trochlear or fourth nerve (or patheticus) spring from the cells of a nucleus which lies in the grey substance of the floor of the cerebral aqueduct in hne with the oculo-motor nucleus, but in the region of the inferior quadri-geminale bodies.
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The medial branches of the posterior primary divisions of all the lumbar nerves end in the multifidus spinse and those of the three lower nerves send very small branches to the skin of the sacral region.
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The posterior primary divisions of the upper four sacral nerves escape from the vertebral canal by passing through the posterior sacral foramina; those of the fifth sacral nerve pass out through the hiatus sacralis between the posterior sacro-coccygeal ligaments.
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The posterior primary divisions of all the thoracic nerves divide into medial and lateral branches while in the vertebral groove.
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