200 ELEMENTARY PHYSIOLOGY less. 



kidneys diminishes, and the reverse is observed in cold 

 weather. 



This power of mutual substitution, however, only goes 

 a little way ; for if the kidneys be extirpated, or their 

 functions much interfei'ed with, death ensues, however 

 active the skin may be. And, on the other hand, if the 

 skin be covered with an impeneti-able varnish, the tempe- 

 rature of the body rapidly falls, and death takes place, 

 though the lungs and kidneys remain active. 



10. The Secretion of Sweat and its Nervous 

 Control.— In analysing the process by which the per- 

 spiration is eliminated from the body, it must be 

 recollected, in the first place, that the skin, even if there 

 were no glandular structures connected with it, would be 

 in the position of a moderately thick, permeable mem- 

 brane, interjjosed between a hot fluid, the blood, and the 

 atmosphere. Even in hot climates the air is, usually, far 

 from being completely saturated with watery vapour, and 

 in temperate climates it ceases to be so saturated the 

 moment itcomes into contact with the skin, the temperature 

 of which is, ordinarily, twenty or thirty degrees above its 

 own. . 



A bladder exhibits no sensible pores ; but if a bladder 

 be filled with water and suspended in the air, the water 

 will gradually ooze through the walls of the bladder, and 

 disappear by evaporation. Now, in its relation to the 

 blood, the skin is such a bladder full of hot fluid. 



Thus, persjjiration to a certain amount, must always be 

 going on through the substance of the integument, but 

 probably not to any great extent ; though what the 

 amount of this perspiration may be cannot be accurately 

 ascertained, because it is entirely masked by the secretion 

 from the sweat-glands. 



When from any ordinary cause an increased formation 

 of sweat takes place, two things usually happen. The 

 small arteries which supply the capillary network 

 surrounding the coiled tube of the sweat-gland dilate 



