FUNCTIONS OF THE BRAIN 945 



third possibility would be that 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 specialization, combined with a tendency to development of 

 central specialization. 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 in 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 

 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 associated 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. 



Acquisition of Co-ordination of Voluntary Movements. The co-ordina- 

 tion of movements has already been alluded to in connection with the 

 spinal reflexes. No fundamental distinction can be drawn between the 

 co-ordination of reflex and of voluntary movements, but the conscious 

 and often long-continued efforts necessary to acquire mastery over the 

 latter lends to their co-ordination a special interest. The new-born child 

 brings with it into the world a certain endowment of co-ordinative 

 powers; it has inherited, for example, from a long line of mammalian 

 ancestors the power of performing those movements of the cheeks, lips, 

 and tongue, on which sucking depends; perhaps from a long, though 

 somewhat shadowy, race of arboreal ancestors the power of clinging 

 with hands and feet, and thus suspending itself in the air. Many move- 

 ments, such as walking and the co-ordinated muscular contractions 

 involved in standing, and even in sitting, which, once acquired, appear 

 so natural and spontaneous, have to be learnt by painful effort in the 

 hard school of (infantile) experience, and this despite the fact that in 

 these movements the voluntary co-ordination mechanism makes use to 



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