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 vagus or pneumogastric nerves are the longest of the cranial nerves, and they are remarkable for their almost vertical course, their asymmetry, and their extensive distribution, for, in addition to supplying the lung and stomach, as the name ' pneumo-gastric ' indicates, each nerve gives branches to the external ear, the pharynx, the larynx, the trachea, the oesophagus, the heart, and the abdominal viscera. They are commonly referred to as the tenth pair of cranial nerves.
The spinal cord [medulla spinalis] is the lower (caudal) and most attenuated portion of the central nervous system. It is approximately cylindrical in form and terminates conically. Its average length in the adult is 45 cm. (18 in.) in the male and 42 cm. in the female. It weighs from 26 to 28 grams or about 2 per cent, of the entire cerebro-spinal axis.
After birth it grows more rapidly and for a longer period than the encephalon, increasing in weight more than sevenfold, while the brain increases less than half that amount. Its specific gravity is given as 1.038.
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.
The fibers of the optic nerve are the central processes of the ganglion cells of the retina. Within the ocular bulb they converge to the optic papilla, where they are accumulated into a rounded bundle, the optic nerve.
The oculo-motor or third cranial nerve is a purely motor nerve. Each supplies seven muscles connected with the eye, two of which, the sphincter of the iris and cihary muscle, are within the ocular bulb.
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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