268 UNCONSCIOUS NERVOUS OPERATIONS 



evaporation, is affected by the vasomotor and nervous 

 mechanisms of the skin. 



Variation in production of heat. The processes of diges- 

 tion are attended by the setting free of heat. The 

 temperature rises after a meal, while a marked condition 

 in starvation is the fall of the bodily temperature. Mus- 

 cular contraction always going on develops heat, and the 

 more active the muscles are, the greater is the amount 

 of heat produced. 



There is also evidence, from numerous experiments, 

 of direct nervous control over the production of heat. 

 Afferent impulses from the skin or other organs reach the 

 central nervous system and some restricted " heat center " 

 not yet anatomically made out in the central part of the 

 brain. The exact path of these impulses is not yet ascer- 

 tained. The stimulation of the heat center, wherever it 

 may be placed, gives rise to efferent impulses by which 

 activity in the tissues is increased and heat is produced. 



395. Clothing. Though the body is able to endure a 

 large amount of exposure to heat and cold without injury, 

 yet the mechanism for heat production may be overtaxed, 

 as well as the digestive, the muscular, or the nervous 

 system. Any such overtaxing interferes with the other 

 functions of the body. Excessive exposure to cold, or 

 insufficient clothing, forces the body to use an undue 

 amount of energy in manufacturing heat, and other parts 

 of the vital economy suffer ; growth in the young is inter- 

 fered with, and mental and muscular effort become diffi- 

 cult. The clothing worn should therefore be such as will 

 assist in preserving the natural' temperature of the body, 

 and the amount and the material will vary with climate 

 and season, as well as with the age, habits, and health 

 of the wearer. 



