. Animal physiology. Physiology, Comparative; Physiology, Comparative. GENERAL STRUCTURE AND ACTIONS OF NERVOUS SYSTEM. 327 instrument of the mind. Through its means, the individual becomes conscious of what is passing around him ; its operations are connected, in a manner we are totally unable to explain, with all his thoughts, feelings, desires, reasonings, and determi- nations; and it communicates the influence of these to his muscles, exciting them to the operations which he desires or determines to execute. But of itself it cannot produce any movement, or give rise to any action ; any mor
. Animal physiology. Physiology, Comparative; Physiology, Comparative. GENERAL STRUCTURE AND ACTIONS OF NERVOUS SYSTEM. 327 instrument of the mind. Through its means, the individual becomes conscious of what is passing around him ; its operations are connected, in a manner we are totally unable to explain, with all his thoughts, feelings, desires, reasonings, and determi- nations; and it communicates the influence of these to his muscles, exciting them to the operations which he desires or determines to execute. But of itself it cannot produce any movement, or give rise to any action ; any more than the expansive force of Steam could set a mill in motion, without the machinery of the Steam-Engine for it to act upon. The Muscular System is the apparatus by which the movements of the body are immediately accomplished; and these it effects by the peculiar power it pos- sesses, of contracting upon the application of certain stimuli, of which Nervous Agency is the most powerful. General Structure and Actions of the Nervous System. 428. The Nervous tissue consists of two distinct structures; of one of which the trunks of the nerves a are entirely made up ; whilst the other enters largely into the composition of the ganglia or centres of action (§.65). The former, termed the white or fibrous tissue, consists of straight fibres, lying side by side, and bound together by areolar tissue into bundles; these, again, are united with others, into a larger group ; and by the union of a considerable number of such groups, the nervous trunks are formed, which are distributed through the body, especially to the skin and muscles. These fibres, however, differ entirely in their character from any which have been hitherto described. They are found, when examined with a good Microscope, to be tubes, containing a sort of pulpy substance, which may be squeezed from their ends when they are cut across. Each of these tubes runs its course completely separated from the rest, no union or. Fig
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