CLARK, G. 
area shown in the previous slide, that is, the lesion is largely in the 
area most sensitive to heat. It is also obvious that the lesion is 
slightly asymmetrical and also incomplete. The heat load test for 
these animals was an exposure to 40 C in a thermostatically con- 
trolled box with the animal confined to a reasonably comfortable 
hammock. Under these conditions a normal cat's temperature will 
rise rapidly and panting soon ensues. The average rectal temper- 
ature at which panting occurs is 39.7 C which is about 0.8 C 
above the normal temperature for a cat. The effects of lesions 
similar to the one previously shown at various levels in the anterior 
hypothalamus and anterior to the anterior hypothalamus are sum- 
marized in Figure 4. In cat 53 the lesion was anterior to the heat 
sensitive area while in the other cats the lesions were either in the 
region sensitive to heating or caudal to it. In these remaining 
seven animals in heat load tests made one month or more after the 
operation there was little or no change in respiratory rate even 
though their body temperatures rose markedly. In all but one of these 
the respiratory rate was less than 40 per minute even when the 
body temperature was elevated to 41.1 C at which level the tests 
were concluded. The results of these tests, then, are in perfect 
agreement with the localization obtained with local heating of the 
brain. Destruction of the heat sensitive area results in an inability 
to withstand heat. Figure 5 shows tests in a series of cats with 
lesions in the lateral portion of the caudal hypothalamus. These 
animals showed the same loss in ability to tolerate an acute heat 
load as seen inthe cats with anterior lesions. The animals, however, 
also had marked falls in rectal temperature when exposed to a 
moderately cool environment. In similartests the cats with anterior 
lesions withstood a cold load as well as normal animals. Although 
some of these animals with caudal lesions were tested two months 
or more after their operation, they showed no improvement in abil- 
ity to withstand heat; however, there was one group of cats which 
had only a temporary loss. These are shown in Figure 6. Here are 
two groups of cats with small lesions in the anterior portion of the 
lateral hypothalamus. In heat load tests one week postoperatively 
similar results were obtained in both groups. The slides show re- 
sults obtained in tests one month after the operation. It will be seen 
that the cats with asymmetrical lesions withstood the acute heat 
load with only slightly more difficulty than normal animals while 
those with symmetrical lesions were unable to withstand the heat. 
Presumably in the cats with asymmetrical lesions there had been 
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