SPINAL REFLEXES. 843 



meets the observer than this, nor more often disturbs his observations 

 on other points. It brings home to the imagination the constant mani- 

 fold factors that must be playing upon the spinal cord, even under 

 apparently the most uniform conditions. The play of the external 

 environment upon organ is more or less under control of the experi- 

 menter. It is the varying condition of the central organ and of its 

 bodily environments which creates his difficulties. 



Under strychnin poisoning the whole skeletal musculature may be 

 thrown into action from any one point of skin, and even when all the 

 afferent roots have been severed, except that particular one conducting 

 from the sole point excited. 1 Each local afferent path must evidently lead 

 to not merely one but many, and ultimately to the whole motor mechanism. 

 The afferent path from a digit is thus, potentially at least, connected with 

 the whole motor organ of the individual. Yet harmonious co-ordination 

 of the contraction of the individual muscles in any functional group, 

 and the harmonious mutual co-operation of functionally related muscle 

 groups, characterises the natural movements of a normal animal. This 

 mechanism has been symbolised by supposing in the cerebro-spinal 

 organ between the various central connections open to the afferent 

 path, resistances so graded and adjusted that only harmonious motor 

 units and unit-groups become excited, and these each only in that 

 degree due for execution of appropriate movement. It might have 

 been supposed that with the cerebro-spinal organ maimed by subtrac- 

 tion from it of the entire brain, any afferent path entering the central 

 nervous system, and there connected with not one but many potentially 

 all the motor spinal units, would, the guidance of the brain and its 

 great sense organs gone, act on the motor units both synergic and 

 antergic so indiscriminately as to induce, if strongly stimulated, complete 

 discharge of all of them, or, if weakly stimulated, feeble or incomplete 

 discharge of all of them. Such discharges would of course produce 

 unnatural movements, in which the action of one muscle or set of 

 muscles would impair or defeat the action of another muscle or set of 

 muscles. As a fact, the reactions of the skeletal musculature of the 

 spinal animal display no inco-ordination of this kind. The afferent path 

 evokes discharge only of harmoniously related motor groups. If it be 

 stimulated strongly, the number of motor groups excited may be extensive 

 and their situation involve the whole length of the spinal axis ; but the 

 groups excited contemporaneously are always harmoniously acting groups, 

 e.g. flexors of hip with flexors of knee, and not groups mutually subversive 

 of each other's action. Each individual muscle of each group, so far as 

 analysed, seems in its action balanced and subordinated as in a 

 " natural " movement. Nor does progressive weakening of the stimu- 

 lation of the afferent path lead to the appearance of inco-ordination, as 

 might have been expected by the excitation of the various groups 

 becoming irregular and partial. As a fact, it leads only to the non- 

 appearance of the reaction in a progressively increasing number of the 

 harmoniously acting regions of the musculature. No more striking 

 contrast can be cited than that holding between this and the effect of 

 excitation of a motor spinal root or of a filament of a motor root. The 

 latter throws into contraction simultaneously muscles whose action is 

 opposed ; what movement results will be determined merely by which 

 muscle of those in action pulls the hardest or has the better leverage. 



1 See Vulpian, " Le9ons sur la physiologic du systeme nerveux," Paris, 1866. 



