332 SECRETION. 



evaporates, and in assuming the gaseous form causes so much heat 

 to become latent that the cutaneous surfaces are cooled down to 

 their natural temperature. 



So long as the air is dry, so that evaporation from the surface 

 can go on rapidly, a very elevated temperature can be borne with 

 impunity. The workmen of the sculptor Chantrey were in the 

 habit, according to Dr. Carpenter, of entering a furnace in which 

 the air was heated up to 350 ; and other instances have been known 

 in which a temperature of 4'00 to 600 has been borne for a time 

 without much inconvenience. But if the air be saturated with 

 moisture, and evaporation from the skin in this way retarded, the 

 body soon becomes unnaturally warm ; and if the exposure be long 

 continued, death is the result. It is easily seen that horses, when 

 fast driven, suffer much more from a warm and moist atmosphere 

 than from a warm and dry one. The experiments of Magendie and 

 others have shown 1 that quadrupeds confined in a dry atmosphere 

 suffer at first but little inconvenience, even when the temperature 

 is much above that of their own bodies ; but as soon as the atmo- 

 sphere is loaded with moisture, or the supply of perspiration is ex- 

 hausted, the blood becomes heated, and the animal dies. Death 

 follows in these cases as soon as the blood has become heated up to 

 8 or 9 F., above its natural standard. The temperature of 110, 

 therefore, which is the natural temperature of birds, is fatal to quad- 

 rupeds ; and we have found that frogs, whose natural temperature 

 is 50 or 60, die very soon if they are kept in water at 100 F. 



The amount of perspiration is liable to variation, as we have 

 already intimated, from the variations in temperature of the sur- 

 rounding atmosphere. It is excited also by unusual muscular 

 exertion, and increased or diminished by various nervous condi- 

 tions, such as anxiety, irritation, lassitude, or excitement. 



4. THE TEARS. The tears are produced by lobulated glands 

 situated at the upper and outer part of the orbit of the eye, and 

 opening, by from six to twelve ducts, upon the surface of the con- 

 junctiva, in the fold between the eyeball and the outer portion of 

 the upper lid. The secretion is extremely watery in its composition, 

 and contains only about one part per thousand of solid matters, 

 consisting mostly of chloride of sodium and animal extractive 

 matter. The office of the lachrymal secretion is simply to keep the 



1 Bernard, Lectures on the Blood. Atlee's translation, Phila., 1854, p. 25. 



