760 A MANUAL OF PHYSIOLOGY 



the ' transposed ' centres, responding at first feebly or not at all to 

 the new impulses, might, by slow degrees, become more and more 

 excitable to them. This would correspond to a peripheral specializa- 

 tion, combined with a tendency to development of central specializa- 

 tion. And, indeed, it is not easy to conceive in what way, except 

 as the result of differences in the nature of impulses coming from 

 the periphery, specialization of sensory areas in the central nervous 

 system could have at first arisen. 



Degree of Localization in Different Animals. Before leaving this 

 subject, two points ought to be made clear : (i) The degree of 

 localization of function on the cortex goes hand in hand with 

 the general development of the brain. In man and the monkey, 

 the motor localization 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 parti- 

 cularly studied in recent years by Mann and Mills, localization is stili 

 less advanced than in the dog. Towards the bottom of the mam- 

 malian 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 associated with the 

 same movements. Taking the position of the centre for the orbi- 

 cularis oculi as a test, Ziehen has lately come to the conclusion that 

 in the anthropomorphous 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 applica- 

 tion of a stimulus and the signal which announces that it 

 has been felt (reaction time) is measured, it is found that the 

 cerebral processes associated with the perception of the 

 simplest sensation and the production of the simplest 

 voluntary contraction is longer than the time which 

 the spinal centres require for the elaboration of even com- 

 plex 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 oi 



