400 THE SPINAL CORD. 



tered for the relief of neuralgia, that the pain is completely abolished 

 by the influence of the drug, while the patient retains completely 

 his consciousness and his ordinary sensibility. 



In all cases, however, if the influence of the narcotic be pushed 

 to its extreme, both kinds of sensibility are suspended together, and 

 the patient becomes entirely unconscious of external impressions. 



MOTION. Wherever muscular tissue exists, in any part of the 

 body, we find the power of motion, owing to the contractility of 

 the muscular fibres. But this power of motion, as we have already 

 seen, is dependent on the nervous system. The excitement which 

 causes the contraction of the muscles is transmitted to them by the 

 nervous filaments ; and if the nerve supplying a muscle or a limb 

 be divided or seriously injured, these parts are at once paralyzed 

 and become incapable of voluntary movement. A nerve which, 

 when irritated, acts directly upon a muscle, producing contraction, 

 is said to be excitable and its excitability, acting through the mus- 

 cle, produces motion in the part to which it is distributed. 



The excitability of various nerves, however, often acts -during 

 life upon other organs, beside the muscles ; and the ultimate effect 

 varies, of course, with the properties of the organ which is acted 

 upon. Thus, the nervous excitement transmitted to a muscle pro- 

 duces contraction, while that transmitted to a gland produces an 

 increased secretion, and that conveyed to a vascular surface causes 

 congestion. In all such instances, the effect is produced by an 

 influence transmitted by the nerve directly to the organ which is 

 called into activity. 



But in all the external parts of the body muscular contraction 

 is the most marked and palpable effect produced by the direct 

 influence of nervous excitement. We find, therefore, that so far 

 as we have yet examined it, the nervous action shows itself princi- 

 pally in two distinct and definite forms ; first, as sensibility, or the 

 power of sensation, and second, as excitability or the power of pro- 

 ducing motion. 



DISTINCT SEAT OF SENSATION AND MOTION IN THE NERVOUS 

 SYSTEM. Sensation and motion are usually the first functions 

 which suffer by any injury inflicted on the nervous system. As a 

 general rule, they are both suspended or impaired at the same time, 

 and in a nearly equal degree. In a fainting fit, an attack of apo- 

 plexy, concussion or compression of the brain or spinal cord, or a 



