498 OF THE NERVOUS SYSTEM AND ITS ACTIONS. 



to the will; whilst the upper remains under the control of the animal, 

 as completely as before. Now it is not conceivable that, in this last 

 case, sensation and volition should exist in that portion of the spinal 

 cord, which remains connected with the nerves of the posterior extremi- 

 ties, but which is cut off from the brain. For if it were so, there must 

 be two distinct centres in the same animal, the attributes of the brain 

 not being affected ; and, by dividing the spinal cord into two or more 

 segments, we might thus create in the body of one animal two or more 

 distinct centres of sensation, independent of that which still holds its 

 proper place in the Encephalon. To say that two or more distinct 

 centres of sensation are present in such a case, would really be in effect 

 the same as saying, that there are two or more distinct minds in one 

 body, which is manifestly absurd. 



878. But the best proofs of the limitation of the endowments of the 

 Spinal Cord, are derived from the phenomena presented by the Human 

 subject, in Cases where that organ has suffered injury, by disease or 

 accident, in the middle of the back. We find that, when this injury has 

 been severe enough to produce the effect of a complete division of the 

 Cord, there is not only a total want of voluntary control over the lower 

 extremities, but a complete absence of sensation also, the individual 

 not being in the least conscious of any impression made upon them. 

 When the lower segment of the Cord remains sound, and its nervous con- 

 nexions with the limbs are unimpaired, distinct reflex movements may 

 be excited in the limbs, by stimuli directly applied to them, as, for in- 

 stance, by pinching the skin, tickling the sole of the foot, or applying 

 a hot plate to its surface ; and this without the least sensation, on the 

 part of the patient, either of the cause of the movement, or of the move- 

 ment itself. This fact, taken in connexion with the preceding experi- 

 ments, both upon Vertebrated and Articulated animals, distinctly proves 

 that Sensation is not a necessary link in the chain of reflex actions ; but 

 that all which is required is an afferent fibre, capable of receiving the 

 impression made upon the surface, and of conveying it to the centre ; a 

 ganglionic centre, composed of vesicular nervous substance, into which 

 the afferent fibre passes ; and an efferent fibre, capable of transmitting 

 the motor impulse, from the ganglionic centre, to the muscle which is 

 to be thrown into contraction. 



879. These conditions v are realized in the Spinal Cord. We may 

 have reflex actions excited through any one isolated segment of it, as 

 through a single ganglion of the ventral cord of Articulata ; but they 

 are then confined to the part supplied by the nerves of that segment. 

 Thus, if the spinal cord of a Frog be divided just above the origin of 

 the crural nerves, the hind-legs may be thrown into reflex contraction 

 by various stimuli applied to themselves ; but the forelegs will exhibit 

 no movement of this kind. But when the brain has been removed, and 

 the Spinal Cord is left entire, movements may be excited in distant 

 parts, ^as, for example, in the fore-legs, by any powerful irritation of the 

 posterior extremities, and vice versd. This is particularly well seen in 

 the convulsive movements, which take place in certain disordered states 

 of the nervous system ; a slight local irritation being sufficient to throw 

 almost any muscle of the body into a state of energetic action ( 885). 



