Evolution.

Society for Experimental Biology (Great Britain)

Date:: 1953

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Evolution. Source: Wellcome Collection.

461/484 (page 429)

$SOCIAL BEHAVIOUR AND PRIMATE EVOLUTION 43I order than that of the areas concerned with special senses and motor function. At present, comparative data is lacking to enable us to say whether this fact, on which Fulton (1949) lays so much emphasis from a functional point of view, has any evolutionary significance, i.e. represents any exceptional enlargement of these functions in man in comparison with other primates and mammals. Nevertheless, the social circumstances in which the evolu¬ tion of the primates must, by inference, have taken place is just such as to require the development of a differentiated control of autonomic functions. In particular, since the hypothalamus functions as a centre concerned with the reflex maintenance of the internal environment, and also contains the centres which are the source of efferent impulses, providing the expres¬ sion of specific patterns of emotional behaviour, e.g. rage and sleep (involving as they do major shifts in the constitution of the internal environment), both aspects of its function will have to be brought under the control of the cortex. Here reside the neurons with definite pathways, bringing about localized and circumscribed autonomic changes in parts and organs of the body concerned with particular functions (Fulton, 1949). There are, however, other aspects to the question of the adaptations necessary for life in the primate social environment. Equilibration behaviour tends to lead the maturing male primate away from situations liable to evoke emotional responses in him. However, under the extreme provocation exerted by the trigonal situation, the ability to diminish emotive responses, e.g. rage and fear reaction, would thus minimize the chances of impairment to his breeding potential. It would be reasonable, therefore, to expect that, during primate evolution, enlargement of those structures mediating the control of emotive responses would take place. In this connexion, the work of Bard & Mountcastle (1948) focuses our attention on the amygdaloid nucleus and its connexions. These authors point out that when the neocortex is removed from cats, leaving intact allocortical and mesocortical structures (amygdala, hippocampus, pyriform lobe and cingular gyrus), a state of extreme placidity or refractiveness to rage-provoking stimuli develops. On the further removal of midline cortex (area 24), or by further ablation of pyriform lobe, amygdala and hippocampus, the slightest stimulus provokes intense rage. An extra¬ ordinary and lasting depression of rage responses was produced by bilateral removal of the amygdala and much of the pyriform lobe. From this work it appears that the amygdala is a centre maintaining a continual state of inhibition on the hypothalmic centres concerned with rage responses in the cat, being innervated from mesocortical, allocortical and neocortical structures. In the macaque electrolytic lesions in the region of the amygdala produces long-lasting placidity (E. A. Turner, 1952, private communica-$

Evolution.

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