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
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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