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 rnutilatio» of the body. The power itself is no doubt more energetic, as the muscular power is, but it is less lasting.
In the articulate classes movements of pre cisely the same nature may be observed. fhe common earthworm may be divided into seve ral pieces, and each piece will continue to writhe so long as the iiritation 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 ganglionic chain of the articulata is analogous, not, as formerly sup posed, to the sympathetic system, but to the cerebro-spinal centres of Vertebrate. In both the Vertebrata and the Invertebrate Articulata each segment of the body is provided with its proper ganglionic centre, whiclt 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, exhibitirig 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 excitinr, these movements as much as when the stimurus 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 midi circum stances no motion can be excited by stiniulation 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.