THE MECHANISM OF CO-ORDINATED MOVEMENTS 380 



hind limbs hang freely, these latter execute a series of alternate move- 

 ments of flexion and extension. The starting-point of these is the 

 stretching of the anterior thigh muscles. Once started they continue 

 of themselves, each act exciting the alternating antagonistic act. 



A reflex act has often been distinguished from other reactions, 

 described as conscious or purposive, by its fatality i.e. by the 

 invariability with which it results on a given stimulus, whether the 

 reaction be for the good of the animal as a whole or not. Thus a 

 decapitated eel will wind itself with equal readiness around a stick or a 

 hot poker. All reactions are, however, purposive. The machinery for 

 them has been evolved and the paths laid down in the spinal cord under 

 the action of natural selection, so that they must act, at any rate in the 

 average of cases, towards the well-being of the animal as a whole. 

 Since the nerve path involved in any reaction includes a number of 

 synapses, each of which may be influenced from other parts of the 

 body in a positive or negative direction, an absolute uniformity of 

 response cannot be predicated for any one reaction. There will be 

 changes in the facility with which it is evoked and changes in its 

 extent, and these will become the more operative the greater the 

 complexity of the arc, and the larger the number of other impulses 

 to which it may be subject. The fatality of response is therefore only 

 shown at its best in the very simplest of reflexes, or the most lowly 

 organised nervous systems. 



The purposive character of the reflexes obtained from the spinal frog has 

 sometimes led writers, especially in pre-Darwinian days, to endow the spinal 

 cord with a guiding intelligence. At the present time we recognise that evTy 

 reaction of a living being must be purposive, in the sense of being adapted to 

 the preservation of the species, if the latter is to survive in the struggle for 

 existence. The question as to whether we are justified in predicating the 

 existence of even a germ of consciousness or volition in the spinal animal must 

 be decided in the negative. " Associative memory would seem to be a postulate 

 for the very existence of perception. Where even simplest ideas are not, there 

 cannot be consciousness. Animal movements that are appropriate not only 

 for an immediate but also for a remote end indicate associative memory. The 

 approach of a dog in answer to the calling of its name, the return of an animal 

 when hungry to the place where it has been wont to receive food, such move- 

 ments may be taken as indicative of consciousness since they indicate the 

 working of associative memory. Examined by this criterion all purely spinal 

 reactions fail to evince features of consciousness " (Sherrington). 



THE PART PLAYED BY AFFERENT IMPRESSIONS IN THE CO- 

 ORDINATION OF MUSCULAR MOVEMENTS. Every reflex act is 

 initiated in the first place by some form of sensory stimulus. In the 

 carrying out of the muscular contractions and the resultant movements 

 of the limbs, other impulses are set up in the structures which subserve 

 deep sensibility, including those of muscles, which in their turn affect 

 the excitability and the activity of the" motor neurons. These 



