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 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.
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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.
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The hypoglossal nerves are exclusively motor; they supply the genio-hyoidei and the extrinsic and intrinsic muscles of the tongue except the glosso-palatini. They are usually designated as the twelfth pair of cranial nerves.
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The glosso-pharyngeal or ninth cranial nerves are mixed nerves and each is attached to the medulla by several roots which enter the posterolateral sulcus, dorsal to the anterior end of the olivary body and in direct line with the facial nerve.
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The Sympathetic Ganglia of the Head and Their Associations with the Cranial Nerves
The sympathetic system of the head, like that of the remainder of the body described below, is arranged in the form of a continuous gangHated plexus subdivided into sub-plexuses.
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The vestibular nerve is purely sensory. With the peripheral processes of its cells of origin terminating in the neuro-epithelium of the semicircular canals and the vestibule, and their central processes conveying impulses which are distributed to the gray substance of the cerebellum and spinal cord, the nerve comprises a most important part of the apparatus for the equilibration of the body.
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