418 OF SECRETION. 



C \ 



of glandulse in other parts of the skin is sometimes greater, but gene- 

 rally less than this ; and, according to Mr. Wilson, about 2800 may be 

 taken as the average number of pores in each square inch throughout 

 the body. Now the number of square inches of surface, in a man of 

 ordinary stature, is about 2500; the number of pores, therefore, is 

 seven millions ; and the number o* ? inches of perspiratory tubing would 

 thus be 1,750,000, or 145,833 feet, or 48,611 yards, or nearly 28 

 miles. 



744. From this extensive system of glandulse, a secretion of watery 

 fluid is continually taking place ; and a considerable amount of solid 

 matter also is drawn off by the epithelium-cells that line the tubuli. 

 Under ordinary circumstances, the fluid is carried off in the state of 

 vapour, forming the insensible perspiration ; and it is only when its 

 amount is considerably increased, or when the surrounding air is already 

 so loaded with moisture as to be incapable of receiving more, that the 

 fluid remains in the form of sensible perspiration upon the surface of 

 the skin. It is difficult to estimate the proportion of solid matter con- 

 tained in this secretion ; partly on account of the great variations in 

 the amount of fluid eliminated by the Sudoriparous glands, which are 

 governed by the temperature of the skin ; and partly because the secre- 

 tion can scarcely be collected for analysis free from the sebaceous and 

 other matters which accumulate on the surface of the skin. According 

 to Anselmino it varies from J to 1J per cent. ; and consists in part of 

 lactic acid, to which the acid reaction and sour smell of the secretion 

 are due ; in part of a proteine-compound, which is probably furnished by 

 the epithelium-cells that line the tubes ; and in part of saline matters, 

 directly proceeding from the serum of the blood. Urea has been re- 

 cently detected in the perspiration of the inhabitants of warm climates. 



74o. The amount of* fluid excreted from the skin is almost entirely 

 dependent upon the temperature of the surrounding medium ; being in- 

 creased with its rise, and diminished with its fall. The object of this 

 variation is very evident ; being the regulation of the temperature of 

 the body. When the surface is exposed to a high degree of external 

 heat, the increased amount of fluid set free from the perspiratory glands 

 becomes the means of keeping down its own temperature ; for this fluid 

 is then carried off in a state of vapour, as fast as it is set free ; and 

 in its change of form, it withdraws a large quantity of caloric from the 

 surface. But if the hot atmosphere be already loaded with vapour, this 

 cooling power cannot be exerted ; the temperature of the body is raised, 

 and death supervenes, if the experiment be long continued. The cause 

 of the increased secretion is probably to be looked for in the increased 

 determination of blood to the skin, which takes place under the stimulus 

 of heat: The entire loss by Exhalation from the lungs and skin, during 

 the twenty-four hours, seems to average a little above 2 Ibs. In a warm 

 dry atmosphere, however, it has been found to rise to as much as 51b. 

 whilst in a cold damp one, it may be lowered to If Ib. Of this quantity, 

 the pulmonary exhalation is usually somewhat less than one-third, and 

 the cutaneous somewhat more than two-thirds ; but when the quantity 

 of fluid lost is unusually great, the increase must be chiefly in the Cuta- 

 neous exhalation ; since, as already pointed out ( 701), the amount of 



