skin and body temperature and must therefore remove more heat by evaporation. The 

 earlier and greater output by the sweat glands of acclimatized individuals has been 

 noted in many laboratories to occur on the first 7-14 days of continuous or repeated 

 exposure. It may be detected even after a year of living in hot climates. In desert 

 conditions where sweat is so readily evaporated it certainly appears to be an adjust- 

 ment of real importance. 



It should be repeated that we have no first hand data of the indigenous desert 

 peoples or on those who have lived long periods in desert regions. 



5. Breakdown 



Enough has been said to indicate that adjustment to severe desert conditions is 

 physiologically of a complex nature and it is not surprising that this may fail at dif- 

 ferent points. There is also great individual variation in liability to these failures of 

 physiological adaptation, at least in Europeans, for information on indigenous peoples 

 is lacking. Brief consideration of these disorders will serve to indicate the nature of 

 the physiological breakdown. 



a. Circulatory failure with syncope occurs in most heat disorders but in a simple 

 form it is often the outcome of insufficient acclimatization and training for hard 

 work at high levels. In deserts it is often the sign of incipient dehydration or of 

 salt imbalance. 



b. Dehydration resulting from lack of water in relation to heat load is by far the 

 most serious danger in deserts. The body's ability to deal with water shortage 

 is more limited than its capacity to compensate for salt shortage. The number of 

 days of survival in the desert can be readily calculated when water supply fails. 

 This has been dealt with in a thoroughgoing manner by Adolph and his colleagues 

 who have mapped these survival limits for all desert regions and for different 

 conditions of work allowing for movement either at night or day. There is no 

 evidence at all that individuals can be 'acclimatized' or 'hardened' to a low 

 water level. Enforced reduction in water intake does not reduce the deficit of 

 body water by sweating. Performance in the heat is better if water is taken con- 

 tinually. Fortitude when water is short is to a large extent a matter of morale. 

 The work of Adolph should be consulted for a vivid account of the key importance 

 of water supply not only to survival but to efficiency in day to day work in the 

 desert. Adolph describes many practical ways of reducing water requirements 

 such as the best use of shade and clothing. 



c. Lack of salt may produce relatively mild effects such as undue fatiguability or 

 severe cramp of the abdomen and limb muscles. As already indicated, there is 

 good physiological compensation for low salt intake but it is likely that some 

 individuals are far less efficient in their ability to conserve salt by reducing its 

 concentration in urine and sweat. The desirability of supplementing salt has al- 

 ready been commented on. 



d. Sweat gland fatigue can be demonstrated in laboratory studies but its exact re- 

 lationship to the apparently complete cessation of sweating seen in the failure 

 of heat regulation with hyperpyrexia, known as 'heat-stroke', is not clear. In 



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