438 



[November, 1011. 



SCIENTIFIC AGRICULTURE. 



THE EFFECT OF THE SUN IN THE 

 TROPICS ON ANIMALS AND MAN. 



By Hans Aron, 

 Professor ot Physiology in the 

 University ot Manila. 



(Berlin klin. Woch., June 9, 1911.) 



(From the Veterinary News, No. 403, 

 Vol, VIII., September 23, 1911.) 



The rays emitted by the sun may be 

 divided into three groups : (a) The ultra- 

 red or heat rays ; (6) the visible or lieht 

 rays ; (c) the ultra-violet or actinic rays. 

 Freer, Bacon, and Gibbs have investi- 

 gated the solar spectrum in Manila, and 

 find that its range on the ultra-violet 

 side is not greater than in Northern 

 regions, but its chemical activity in 

 July as gauged by its action on oxalic 

 acid and uranyl nitrate is from five to 

 twenty times greater than in Chicago. 



Many observers have ascribed the 

 peculiar effects of the tropical sun to 

 the action of the actinic and light rays 

 on the human body. The negative 

 results of the experiments on the 

 American troops in Manila with orange- 

 red coloured clothing as well as numer- 

 ous observations made in the Tropics by 

 Aron have convinced him that the 

 actinic theory cannot be maintained. 



In order to determine the effect of the 

 sun's heat rays in Manila, Aron made a 

 number of experiments, some of which 

 are quoted below. 



One thermometer was placed in the 

 rectum of a dead dog, and another 

 under its skin, and the body suspended 

 in the sun. The thermometer placed 

 under the skin very soon registered a 

 temperature of 46°C, while the one in 

 the rectum showed a gradual rise due 

 to absorption of heat. A living body 

 absorbs heat in the same way, but a rise 

 in the body temperature is prevented by 

 the physiological mechanism for increas- 

 ing the loss of heat. 



A dog kept exposed to tropical sun- 

 shine soon suffers from acute discomfort, 

 and its rectal temperature is found to 

 rise from 38"C. to 39°C. ; a thermometer 

 placed under its skin, however, shows 

 a rise to 40°C. or higher. Rabbits 

 placed under similar conditions show a 

 greater rise of temperature, the ther- 

 mometer placed under the skin record- 

 ing as much as 46C, but when these 

 temperatures are reached the animals 

 soon die. 



The most instructive experiments were 

 those conducted on monkeys (Macacus 

 ■philip., Geoff.). When a strong and 

 healthy monkey is placed, so that it is 

 continuously exposed to the sunshine, 

 its temperature rapidly rises, and 

 seventy to eighty miuutes' exposure, 

 even between 8 and 9 a.m. in December 

 or January, is sufficient to kill it. If 

 protected from the direct rays of the 

 sun it can be kept in the same place for 

 any length of time without suffering 

 any injury. 



Normally the rectal tempei^ature of 

 the monkey is 0"5° to 1°C. higher than 

 its subcutaneous temperature. When 

 exposed to the sun's rays the body 

 temperature rapidly rises, but the sub- 

 cutaneous temperature is always 1° to 

 2°C. above that of the rectum. The 

 immediate effect of shaving a monkey 

 is to reduce its temperature generally, 

 but the effect is more marked on 

 the subcutaneous temperature. When 

 shaved monkeys were exposed to direct 

 rays of the sun, the rise in temperature 

 and subsequent death took place more 

 rapidly than in similar animals whose 

 hair had not been removed. 



That the animals exposed to the sun's 

 rays died as a direct result of hyper- 

 pyrexia is shown by the following experi- 

 ments. Monkeys were exposed to the 

 sun's rays as in the previous experi- 

 ments, but by means of fans a strong 

 current of air was directed on to the 

 animal's body. In spite of several 

 hours' exposure under these conditions 

 the rise in body temperature was only 

 slight, and the animals remained 

 healthy. A control animal exposed 

 within a few yards of the first one, but 

 not artificially cooled, died of hyper- 

 pyrexia in fifty-eight minutes. 



In another experiment the body of a 

 monkey was placed in a wooden box with 

 perforated walls, which was again placed 

 inside a larger one, leaving an air space 

 between the two, so that its head was ex- 

 posed to the sun's rays, but its body was 

 thoroughly protected by the ventilated 

 air space and walls of the boxes. The 

 monkey was kept in this position and 

 exposed to the sun's rays from early 

 morning till night without any rise of 

 body temperature taking place, or any 

 signs of interference with its health. 

 The same animal was exposed under 

 similar conditions to direct sun's rays 

 for altogether fifty-four hours within a 

 period of twelve days, and apparently 

 suffered no damage. During this period 



