702 REFLEX ACTIONS. [Book hi. 



betokening intelligence. Again the ' mechanical ' nature of re- 

 flex 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. 



§ 462. The explanation of the above-quoted instance of 

 apparent choice on the part of the frog's spinal cord is to be 

 sought for in the modifications of the activity of the central 

 mechanism, resulting from afferent impulses other than those 

 which supply the exciting cause of the reflex movement. The 

 existence and importance of these other afferent impulses will 

 be seen if we bear in mind that most reflex movements are, as 

 we have said, ' 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 be- 

 come irregular and ineffectual. We shall have occasion later 

 on in dealing with voluntary movements to point out that the 

 coordination 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 mus- 

 cles. 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 on the ner- 

 vous 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 may take place in the 

 part of the spinal cord which carries out the movement. 



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 



