494 PHYSIOLOGY CHAP. 



perceptibly until the external temperature rises from 29 to 33 C. 

 Between these -limits no formation of sweat drops on the skin is 

 apparent, perspiration maintains its insensible character, and not 

 more than 35 c.c. carbonic acid are given off per hour ( = 9 grrns. 

 per diem). When, on the contrary, the external temperature rises 

 above 33 and reaches 39 C. there is a rapid and progressive 

 increase in C0 2 excretion by the skin. Above 33 perspiration 

 suddenly becomes sensible, i.e. it assumes the form of sweat, and 

 simultaneously with this the amount of C0 2 excreted is suddenly 

 doubled, and increases with the further increase of temperature 

 and secretion of sweat (0'87-1'23 grms. per hour= 20 - 9-29'5 grms. 

 in 24 hours). These results of Schierbeck indicate that the 

 amount of carbonic acid given off by the skin depends not upon 

 the quantity of blood circulating in it, but rather upon the 

 secretory work of the cutaneous glands ; this rises abruptly at the 

 critical temperature of 33 C. to an output of twice the amount 

 of carbonic acid, which thus comes not from the blood directly, 

 but from the sudoriferous glands of which it is a secretion 

 product. 



W. Barratt (1897-99) made a careful study of cutaneous 

 secretion in man under different experimental conditions, by the 

 following method. As in the plethysmograph, the arm was 

 introduced into a metal cylinder made impervious by a rubber 

 sleeve through which air, free from carbonic acid and aqueous 

 vapour, was circulated, and subsequently passed into vessels con- 

 taining sulphuric acid and caustic soda. The metal cylinder was 

 surrounded by a water-bath, which could be heated to various 

 temperatures. Experiments on the normal arm showed that the 

 elimination of carbonic acid is very small in comparison with that 

 of water. The former at a temperature of 35 C. only amounts to 

 0'02 grm. per hour, while that of water = 3-4 grms. The ratio is 

 thus 1-200 ; but it varies from hour to hour and more noticeably 

 from day to day. A ligature applied to the arm produced 

 increased elimination of carbonic acid (up to 40 per cent) and 

 diminution in the elimination of water (up to 20 per cent), which 

 was more pronounced in proportion as the ligature was tighter. 



Varnishing the skin with collodion, by which the orifices of the 

 sweat glands are blocked, causes marked diminution in the 

 elimination of water (to 78'1 per cent) without entirely abolishing 

 it. These results, in conjunction with the direct experiments on 

 the diffusion of water vapour and carbonic acid through thin plates 

 of horn, led Barratt to conclude that the horny layer also played 

 an important part in the elimination of water, and that the dis- 

 charge of carbonic acid through the skin is a process of simple 

 physical diffusion, which takes place through the whole stratum 

 corneum independent of the sweat glands. 



V. Willebrand's researches (1902), on the other hand, agree 



