CLARK, G. 
temporary and reversible damage to the heat sensitive area probably 
as a result of edema which subsided in the first postoperative month. 
If this is the case then it would appear probable that the results in 
the animals with symmetrical lesions were not due to edema. For 
this reason it was assumed that a steady state was present in the 
animals with symmetrical lesions, or in other words, that the 
damage was solely due to loss of tissue and therefore permanent. 
The foregoing was thought to supply a localization of the dual 
centers for temperature regulation. In his text Ranson (1947) sum- 
marized this theory as follows (Figure 7): "On the basis of all the 
available evidence it now seems clear that a center controlling heat- 
loss functions such as panting and sweating is situated in the pre- 
optic region and that a pathway from this center runs backward 
through the lateral hypothalamus. The center for preventing heat 
loss by vasoconstriction and for increasing heat production by 
shivering is situated in the hypothalamus proper; and its descending 
pathway also runs backward through the lateral hypothalamus. 
Both descending pathways run close together dorsolateral to the 
mammillary body and enter the mesencephalic tegmentum. Bilateral 
lesions in the caudal part of the lateral hypothalamus interrupt 
both pathways and interfere withbothheat loss and heat conservation 
mechanisms. Bilateral lesions in the preoptic region destroy the 
heat-loss center leaving the heat conservation center intact; and as 
a result the body temperature either remains normal or may be 
temporarily elevated." 
It has been routine in studies of the effects of various lesions 
on the thermoregulatory ability of experimental animals to utilize 
only external heat or cold loads. There are, however, other con- 
ditions where the dual center theory appears to make definite pre- 
dictions. Upon assumption of a constant work load it is generally 
found that when a steady state is achieved the body temperature is 
at a new and higher level. The dual center theory implies that ani- 
mals with the anterior hypothalamus destroyed would never attain a 
new steady state unless the increased heat production were by 
chance balanced by the increased heat loss occasioned only by the 
increased body temperature. In most cases in such animals a fatal 
hyperthermia should rapidly ensue. As far as I know such experi- 
ments have never been made and yet they are badly needed. 
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