910 REFLEX ACTIONS. [BOOK in. 



over so as to bear on the spot stimulated, and can not be considered 

 as betokening intelligence. Again the ' mechanical ' nature of 

 reflex actions is well illustrated by the behaviour of a decapitated 

 snake. When the body of the animal in this condition is brought 

 into contact at several places at once with an arm or a stick, 

 complex reflex movements are excited, the obvious purpose as well 

 as effect of which is to twine the body round the object. A 

 decapitated snake will however with equal and fatal readiness 

 twine itself round a red-hot bar of iron, which is made to touch its 

 skin in several places at the same time. 



589. In considering the nature of the events in the spinal 

 cord which determine the behaviour of the frog in the instance 

 just mentioned we must bear in mind that the movements in 

 question are 'coordinated;' that is to say not only are many 

 distinct muscles brought into play but certain relations are 

 maintained between the amount, duration and exact time of 

 occurrence of the contraction of each muscle and those of the 

 contractions of its fellow muscles sharing in the movement. In 

 the absence of such coordination the movement would become 

 irregular and ineffectual. We shall have occasion later on in 

 dealing with voluntary movements to point out that the coordina- 

 tion and hence the due accomplishment of a voluntary movement 

 is dependent on certain afferent impulses passing up from the 

 contracting muscles to the central nervous system, and guiding the 

 discharge of the efferent impulses which call forth the contractions. 

 When these afferent impulses affect consciousness we speak of 

 them as constituting a ' muscular sense ; ' it is, as we shall see, by 

 the ' muscular sense ' that we become aware of and can appreciate 

 the condition of our muscles. But we have reason to think that 

 the afferent impulses which constitute the basis of the muscular 

 sense, whatever be their exact nature, in order to play their part in 

 bringing about the coordination of a voluntary movement need 

 not pass right up to the brain and develope a distinct muscular 

 'sense,' but may produce their effect by working 011 the nervous 

 mechanisms of the spinal cord with which the motor fibres 

 carrying out the movement are connected. In other words, the 

 coordination of a voluntary movement takes place in the part of 

 the spinal cord which carries out the movement, and not in the 

 brain, though the latter may be conscious of the whole movement 

 including its coordination. 



But if the spinal cord possesses mechanisms for carrying out 

 coordinated movements, which in the case of voluntary move- 

 ments are discharged by nervous impulses descending from the 

 brain, we may infer that in reflex actions the same mechanisms 

 are brought into action though they are discharged by afferent 

 impulses coming along afferent nerves instead of by impulses 

 descending from the brain. The movements of reflex origin, 

 in all their features except their exciting cause, appear identical 



