872 A MANUAL OF PHYSIOLOGY 



Degree of Localization in Different Animals. Before 

 leaving this subject, two points ought to be made clear : (i) The 

 degree of localization of function in the cortex goes hand in hand 

 with the general development of the brain. In man and the 

 monkey, the motor Ipcalization is more elaborate than in the 

 dog that is to say, a greater number of movements can be 

 associated with definite cortical areas. In the rabbit, whose 

 ' motor ' centres have been particularly studied in recent years 

 by Mann and Mills, localization is still less advanced than in 

 the dog. Towards the bottom of the mammalian group certain 

 ' motor ' areas can still be demonstrated, though they are rather 

 ill-defined, for instance in the hedgehog (Mann), opossum 

 (Cunningham), and ornithorhynchus (Martin). In general the 

 movements of the anterior limb are easier to obtain than those 

 of the posterior. In birds Mills found no evidence of the existence 

 of any ' motor ' centres. 



(2) Areas of the same name (homologous areas) in different 

 groups of animals do not necessarily have the same function 

 that is, in the case of the ' motor ' areas, are not necessarily asso- 

 ciated with the same movements. Taking the position of the 

 centre for the orbicularis oculi as a test, Ziehen has come to the 

 conclusion that in the anthropoid apes and in man, this centre 

 has been pushed forward by the encroachment of the centres 

 behind it, and especially of the visual centre, the arm centre, 

 and the speech centre, which have undergone a great functional 

 development. 



Reaction Time. Just as in a reflex act a certain measure- 

 able time (reflex time) is taken up by the changes that occur in 

 the lower nervous centres, so we may assume that in all psychical 

 processes the element of time is involved. And, indeed, when 

 the interval that elapses between the application of a stimulus 

 and the signal which announces that it has been felt (reaction 

 time) is measured, it is found that for the cerebral processes associ- 

 ated with the perception of the simplest sensation and the pro- 

 duction of the simplest voluntary contraction it is longer than the 

 time which the spinal centres require for the elaboration of even 

 complex and co-ordinated reflex movements. Suppose, e.g., 

 that the stimulus is an induction shock applied to a given point 

 of the skin, and that the signal is the closing of the circuit of 

 an electro-magnet, then, if both events are automatically re- 

 corded on a revolving drum, the interval can be readily deter- 

 mined. It is evident that this includes, not only the time 

 actually consumed in the central processes, but also the time 

 required for the afferent impulse to reach the brain, and the 

 efferent impulse the hand, along with the latent period of the 

 muscles. The time taken up in these three events can be 



