THE STUDY OF TROPICAL SUNLIGHT. 15 



their physical heat regulation is to a much greater extent brought 

 about by water evaporated from the lungs and mouth through 

 increased respiration. The normal subcutaneous temperature 

 of the animals, in the shade, varies from 36 '.6 to 38"^ ; the rectal 

 from 37°.9 to 39°.4. The subcutaneous temperature, therefore, 

 is somewhat below the rectal. However, as soon as the animal 

 is placed in the sun, the subcutaneous temperature rises above 

 the rectal and remains so to the end of the experiment, so that 

 the inside of the body now receives heat from the periphery. 

 The animals exposed to the full sun," without protection or arti- 

 ficial means of lowering the temperature, die in from one hour 

 to one hour and fifty minutes ; the exposures being either in the 

 morning between 10 and 11, or in some cases in the afternoon 

 between 2 and 4, in the months of November and January. Both 

 the skin and rectal temperatures steadily rise during these ex- 

 posures, the maxima before death being 43°. 5 and 42°.7 to 46°. 3 

 and 44°. 8 respectively. 



Entirely different results are obtained if the animals are 

 shaded, even by a small area of shade such as an umbrella or 

 a board, all other conditions being similar, so that the direct 

 rays are excluded, the diffuse rays, excepting those cut off by 

 the shade, still being available. Under these circumstances the 

 skin and rectal temperatures never exceed 40° and the animals 

 remain healthy. Similar results are obtained if the animals are 

 exposed to full insolation, but care is taken to conduct away the 

 excessive heat increment by means of a brisk current of air from 

 a fan. Under these circumstances the subcutaneous and rectal 

 temperatures remain the same as when the animal is shaded, 

 never rising above 40°. 6, and the monkey remains perfectly 

 well. In this last form of experiment the monkey is exposed 

 to all the rays of the sun, including those of lesser refrangibility, 

 heat waves alone being conducted away. If untoward effects 

 are to be attributed to the absorption of the ultra-violet rays, 

 then surely the animal is in the same condition to absorb the 

 latter as he is when no blast of air is present, and their effect 

 should be apparent. On absorption, a large proportion of these 

 rays is presumably converted to heat and conducted away as 

 such, so that it can be assumed that the effects which we observe 

 on exposing these animals to the sun is one of heat, and these 

 conclusions are borne out at autopsy where post-mortem exam- 



" The proportion of the body exposed to the rays in the full sunlight, 

 even toward noon, is the lesser part of the whole, as more than one-half of 

 the body is in its own shade. 



