PHYSIOLOGY OF THE NERVOUS SYSTEM. 



continue their hold for as long as a quarter of 

 an hour or twenty minutes after the removal of 

 the head and the posterior segment of the 

 body. But let the portion of the cord which 

 is connected with the anterior extremities be 

 destroyed, and all such power of movement 

 becomes completely annihilated. 



In birds and mammalia phenomena of this 

 kind are less conspicuous than in the cold- 

 blooded animals, because in them the nervous 

 power becomes extinct so speedily after any 

 mutilation of the body. The power itself is 

 no doubt more energetic, as the muscular 

 power is, but it is less lasting. 



In the articulate clasces movements of pre- 

 cisely the same nature may be observed. The 

 common earthworm may be divided into seve- 

 ral pieces, and each piece will continue to 

 writhe so long as the irritation produced by the 

 subdivision remains, and after that has ceased, 

 movements may be excited in any segment by 

 stimulating its surface: the same phenomena 

 are observable in leeches and various insects. 

 These actions are exactly analogous to those in 

 the segments of the divided body of a verte- 

 brate animal. Each portion of the articulate 

 creature has in its proper ganglion the analogue 

 of the piece of the spinal cord remaining with 

 the segment of the vertebrate animal. These 

 phenomena of function, conjoined with certain 

 anatomical resemblances, make it quite certain 

 that the abdominal gunglionic chain of the 

 articulata is analogous, not, as formerly sup- 

 posed, to the sympathetic system, but to the 

 cerebro-spinal centres of Vertebrata. In both 

 the Vertebrata and the Invertebrate Articulata 

 each segment of the body is provided with its 

 proper ganglionic centre, which is to a certain 

 extent independent of the rest. In the latter, 

 the centres of the segments remain distinct, 

 although connected by fibres which pass from 

 one to the other; but in the former they are as 

 it were fused together at their extremities, and 

 from that fusion results the single cylindrical 

 nervous centre which we call the spinal cord. 



An experiment, to which attention has been 

 directed by Flourens, illustrates very well the 

 difference in the character of the actions of two 

 portions of the spinal cord, according as the 

 brain is connected with or dissociated from it. 

 The spinal cord of an animal is divided about 

 its middle; when the anterior segment (that 

 which still retains its connection with the brain) 

 is irritated, not only are movements of the 

 anterior extremities produced, but the animal 

 evinces unequivocal signs of pain ; when, how- 

 ever, the posterior extremity is irritated, the 

 animal seems not only insensible to pain, but 

 unconscious even of the movements that have 

 been excited in the posterior extremities. If a 

 frog be divided in the back into two segments, 

 the anterior portion crawls about, exhibiting all 

 the indications of sensation and volition ; the 

 posterior segment remains quite motionless un- 

 less some stimulus be applied to it, when 

 movements more or less active may be ex- 

 cited. 



Nothing can be more conclusive than such 

 an experiment, in illustration of the fact that 

 connection with the encephalon is necessary to 



sensation; and that movements, not only with- 

 out volition, but also without consciousness, 

 may be excited by stimulating the segments 

 separated from it. But there is nothing in this 

 experiment to justify the conclusion that during 

 the entire and unmutilated state of the cerebro- 

 spinal axis the mind has no connection with 

 the spinal cord. The experiment only shows 

 that when a portion of that great centre has 

 been removed, the mind retains its connection 

 with the higher or encephalic portion, deserting 

 that which is merely spinal. 



Direct irritation of the spinal cord is capable 

 of exciting these movements as much as when 

 the stimulus is applied to the skin. 



All these motions cease when the spinal cord 

 is removed ; no movement of any kind, volun- 

 tary or involuntary, can then be excited, except 

 by directly stimulating the muscles,or the nerves 

 which supply them, and such movements want 

 the combined and harmonious character which 

 belongs to those which are excited through the 

 nervous centre. 



Division of all the roots of the nerves at 

 their emergence from the spinal cord annihi- 

 lates these movements as completely as the 

 removal of the cord itself. Under such circum- 

 stances no motion can be excited by stimulation 

 of the surface of the body, nor by irritating the 

 cord itself; and this fact may be regarded as 

 an unequivocal proof that the nerves, in ordi- 

 nary actions, are propagators of the change 

 produced by impressions to or from the centres ; 

 and that in the physical nervous actions the 

 stimulus acts, not from one nerve to another 

 directly, but through the afferent nerve upon 

 the centre, which in its turn excites the motor 

 nerve. 



All these facts in the physiological history 

 of the spinal cord lead unequivocally to the 

 following conclusions respecting its office: 

 1. that the spinal cord (that term being used in 

 its simple anatomical sense, the intra-spinul 

 mass) in union with the brain is the instrument 

 of sensation and voluntary motion to the trunk 

 and extremities ; 2. that the spinal cord may 

 be the medium for the excitation of movements 

 independently of volition or sensation in parts 

 supplied by spinal nerves, either by direct 

 irritation of its substance, or by the influence 

 of a stimulus conveyed to it from some surface 

 of the trunk or extremities by its nerves distri- 

 buted upon that surface. 



Of the physical nervous actions of the cord. 

 We must pause here to make a more extended 

 reference to those actions of the spinal cord 

 which are capable of being excited by peri- 

 pheral stimulation, and which are independent 

 of mental change. There is no point in the 

 physiology of the nervous system of more inte- 

 rest or importance than this, inasmuch as these 

 actions are not limited to the cord, but take 

 place in other portions of the cerebro-spinal 

 centre, in which nerves are implanted, and 

 even in ganglions from which nerves take their 

 rise. 



The existence of a class of actions like these 

 has long been known to physicians and physio- 

 logists. By the name of sympathetic actions 

 they excited great interest as to the mode of 



