36 MOTION OF THE JUICES OF THE ANIMAL BODY. 



be followed by a disturbance of this motion, in consequence of which the normal 

 process is changed where this occurs. 



The pressure, which, in consequence of the evaporation, urges the fluids within 

 the body to move towards tHe skin, is, as may readily be understood, equal to the 

 difference of pressure acting on the surface of the skin.* 



From the experiment, Fig. 13, it is plain, that when one of the two surfaces of 

 bladder at the ends of the tube Fig. 12, is exposed to atmospheric evaporation, 

 while the other end is moistened with water, brine, or oil, these liquids are 

 rapidly absorbed by the membrane, that is, are forced in by the external atmospheric 

 pressure, and it is not less obvious, that the same thing takes place with the liquid 

 with which one of the two evaporating surfaces has been moistened in the middle 

 only ; while the evaporation continues around the moistened spot. 



If, therefore, we moisten with a liquid the surface of the evaporating skin at any 

 point, the liquid is forced inwards by the external pressure.! 



Let us suppose any part of the skin to be rubbed with fat, the transpiration 

 ceases at that part.J If now the skin around the part is in its normal activity, if, 

 therefore, in the surrounding parts liquid is constantly passing off by evaporation, 

 the fat must be urged, by the unequal pressure thus arising, towards these parts, or 

 it is absorbed, just as water, in the apparatus, Fig. 12; is absorbed, when in con- 

 sequence of evaporation a difference between the internal and external pressure 

 has arisen. If the whole skin were covered with fat, the absorption would be 

 effected by the pulmonary evaporation. 



The blistering of the skin, and the sun-burning, to which men are exposed at 

 great elevations, arise from the extraordinary dryness of the air, the increased 

 evaporation, and the pressure by which the fluids filling the vessels are forced 

 towards the surface. 



Several causes contribute jointly to the appearance of the sweat, to the efflux of 

 fluid, from the pores of the skin. One of these obviously depends on the velocity, 

 which the fluid set in motion by evaporation or by a mechanical cause, acquires 

 from the accelerated motion of the blood. In consequence of this velocity, the 

 fluid moves out beyond the limits of the absorbing membrane or skin. 



The changes of the vital process, caused by the unequal distribution of fluid in 

 the body in consequence of evaporation, are best seen in animals which live in 

 water, in whom, therefore, the above explained cause of motion in the normal 

 state does not act. When a fish is held immersed in water, so that the head is 

 out of the water, while the rest of the body is covered, it dies in a few minutes. 

 It dies exactly in the same way when head and gills are held in the water, and 

 the body in air (MILNE EDWARDS ;) in both cases, without loss of weight. This 

 fact shows that even if the weight of the animal be kept unaltered by the absorp- 

 tion of water through the body kept in that medium, yet the distribution of the 

 fluids in the body does not take place in the proportion necessary for the preser- 

 vation of their vital functions. The fish dies. 



It is hardly necessary to remind the reader, that the experiments described in 

 the foregoing pages, in so far as they permit us to draw conclusions as to the 

 cause of the motion rf the juices of the animal body, agree in all respects with 

 the observations made on plants by STEPHEN HALES more than 120 years since. || 



The experiments of HALKS on the mechanism of the motion of the sap, may 

 stand as a pattern to all times of an excellent method. That they remain, to this 

 moment, unsurpassed in the domain of vegetable physiology, may be, perhaps 

 explained by the fact that they date from the age of NEWTON. They ought to be 

 familiar to every vegetable physiologis* 



In the beginning of his work, HALES describes the experiments which he made 



The force urging the fluids towards the skin is equal to the difference of pressure acting on 

 the skin. 



t Liquids placed on the skin are absorbed by the evaporation of other parts, 

 j Effect of rubbing fat on a part of the skin or on the whole of it. 

 $ Fishes die in tlr, because the distribution of the fluids is prevented. 

 I S.ipericannts otf HALES on the motion of the sap in plants. 



