ON VOLUNTARY MOVEMENTS. 685 



limited to the area marked out as the pyramidal tract, fibres other than 

 pyramidal fibres would be injured at the same time, since the tract is never 

 a u pure " one. But it has been found that a section of a lateral half of the 

 cord, a lateral hemisectiou, or a section limited to the lateral column of one 

 side, has for one of its principal effects loss of voluntary movement on the 

 same side in the parts supplied by motor nerves leaving the cord below the 

 level of the section. We say " one of its principal effects " because, besides 

 the concomitant interference with sensations concerning which we shall speak 

 presently, the loss of voluntary movement is not absolutely confined to the 

 same side ; there is some loss of power on the crossed side, at least in a large 

 number of cases. We must not lay stress on this crossed paralysis because 

 it is probably one of the effects of the mere operation, not a pure " defi- 

 ciency " phenomenon, and, indeed, appears soon to pass away. But taking 

 into consideration what was said above concerning the effects of removing 

 cortical areas, it is important to note that in the experience of many experi- 

 menters the loss of voluntary power on the operated side diminishes after a 

 while, and that the animal if kept alive and in good health long enough 

 appears to regain almost full voluntary power over the affected parts. In 

 such cases, as in other operations on the central nervous system, there is no 

 regeneration of nervous tissue ; the two surfaces of the section unite by con- 

 nective not nervous tissue, and the tracts which as the result of the section 

 degenerate downward or upward are permanently lost. Hence, even if we 

 admit that in the intact animal a voluntary movement is chiefly carried out 

 by means of efferent impulses passing along the pyramidal tract right down 

 to the motor mechanisms of the cord immediately connected with the motor 

 nerves, we must also admit that the " will " under changed circumstances 

 can find other channels for gaining access to the same mechanisms. 



It has been further observed that if in the dog a hemisection be made at 

 one level, for instance in the lower thoracic region of the cord, and then, 

 after waiting until the voluntary power over the hind limb of that side has 

 returned, a second hemisection, this time on the other side, be made at a 

 higher level, this second operation is followed by results similar to those of 

 the first ; there is loss of voluntary power on the side operated on, with some 

 loss of power on the crossed side, and as in the first case this loss of power 

 not only on the same but also on the crossed side may eventually disappear. 

 This shows among other things that the recovery after the first operation was 

 not due to the remaining pyramidal tract doing the work of both. Further, 

 the hemisection may be repeated a third time, the third hemisection being 

 on the same side as the first, with at least a very considerable return of power 

 over both limbs. That is to say, under such abnormal circumstances volun- 

 tary impulses may, so to speak, thread their way in a zigzag manner from 

 side to side along the mutilated cord until they reach the appropriate spinal 

 motor mechanisms. Such an abnormal state of things does not, however, 

 really militate against the view that under normal circumstances volitional 

 impulses normally travel along the pyramidal tract ; but it does show what, 

 indeed, has already been shown by the phenomena of strychnine poisoning 

 ( 499) that in the central nervous system the passage of nervous impulses 

 (using these words in the general sense of changes propagated along ner- 

 vous material) is not rigidly and unalterably fixed by the anatomical distri- 

 bution of tracts of fibres ; in all such discussions as those in which we are 

 engaged we must bear in mind that physiological conditions as well as ana- 

 tomical continuity are potent in determining the passage of these impulses. 



577. When we reflect on the great prominence of the pyramidal tract 

 in the spinal cord of man as compared with that of the dog, we may justly 

 infer not only that the pyramidal tract is under normal circumstances more 



