THE FUNCTIONS OF THE SKIN 489 



the grey rami communicantes establish connection with the nerves 

 supplying the skin of the various parts of the body (seo p. 750)- 

 After the spinal cord has been divided, sweating does not take place 

 in the parts below the lesion. 



Sweating is provoked in the pads of a cat's foot on stimulating 

 the peripheral end of the sciatic nerve. A few beads of sweat will 

 appear if the nerve be stimulated just after amputation of the foot, 

 but its amount is very scanty in tb.3 absence of blood-flow. The 

 secretory pressure of the sweat, when this is obstructed, rises higher 

 than the blood-pressure. Sweating, therefore, is the result of an 

 active secretion, and not a mere mechanical transudation of fluid 

 from the blood. It is accompanied by an electrical variation in the 

 skin current, as may be demonstrated in the pad of the excised cat's 

 foot, if this be led off to the galvanometer, and the sciatic nerve 

 excited. The nerve-endings in the sweat glands may be paralyzed 

 by atropine, and stimulated by pilocarpine and physostigmine applied 

 locally. 



Transpiration' of water from the bloodvessels through the skin is 

 continually taking place. The skin is thus kept supple and moist. 

 The loss of water by transpiration is insensible perspiration, and it 

 increases with the temperature of the skin. Sensible perspiration is 

 produced by the action of the sweat glands. 



Absorption by the Skin. For a body to be absorbed by the un- 

 broken skin, it is necessary for it either to be of a fatty nature or to 

 be administered in fat. Thus, it is stated that cod-liver oil rubbed 

 into a weakly child serves as a source of nutriment. Mercury has 

 been administered in the form of an ointment. 



Watery fluids are not absorbed. For such a fluid to pass into 

 the tissue lymph, it is necessary to abrade the skin, as in vaccination. 

 A foreign protein injected subcutaneously sensitizes the body, so that 

 a subsequent injection of a trace of the same protein made a few 

 weeks later may produce shock, or death, the phenomena of anaphy- 

 laxis (see p. 111). The mere washing of the uninjured skin by the 

 solution' of foreign protein has no such effect, showing that none is 

 absorbed. 



The Respiratory Function of the Skin. The amount of C0 2 given 

 off by the skin of man is very small. It increases markedly during 

 sweating, and may become two to four times as great as before. This 

 is seen in the following figures obtained from a naked man : 



Temp, of Air. Water per Hour. CO* per Hour. 



29-8 C. 22-2 grammes 0-37 gramme 



31-9 C. 50-3 0-35 



35-4 C. 106-8 1-04 grammes 



38-4 C. 158-3 1-23 



