390 ABSORPTION BY THE SKIN. [BOOK n. 



It does not at present appear certain whether the sweating 

 caused by heat is carried out by direct action on the sweat centres, 

 or by the higher temperature affecting the skin and so producing 

 its effect in a reflex manner ; but in the case of dyspnoea at least 

 we may fairly suppose that the action of the venous blood is 

 chiefly if not exclusively on the nerve centres. Some drugs, such 

 as pilocarpin, which cause sweating, appear to produce their effect 

 chiefly by a local action on the glands since the action continues 

 after the division of the nerves (though pilocarpin at least has as 

 well some action on the nerve centres), and the antagonistic action 

 of atropin is similarly local. Nicotin appears to produce its 

 sweating action chiefly by acting on the central nervous system. 



The sweat-fibres for the hind foot (in the cat) appear to leave 

 the spinal cord by the roots of the last dorsal and first two lumbar 

 or last two dorsal and first four lumbar nerves, pass along the rami 

 communicantes to the abdominal sympathetic, and thus reach the 

 sciatic nerve. Similarly the sweat-nerves for the fore foot leave 

 the spinal cord by the roots of the fourth (or fourth, fifth, and sixth) 

 dorsal nerves, pass into the thoracic sympathetic, thence into the 

 ganglion stellatum, and thus join the brachial plexus ; the course to 

 the foot is finally along the median and ulnar nerves respectively. 

 In the horse and pig the sweat-fibres for the side of face and snout 

 appear to run in branches of the fifth and not in the facial. 



Absorption by the Skin. 



Although under normal circumstances the skin serves only as 

 a channel of loss to the body, it has been maintained that 

 it may, under particular circumstances, be a means of gain. 

 Cases are on record where bodies are said to have gained in weight 

 by immersion in a bath, or by exposure to a moist atmosphere 

 during a given period, in which no food or drink was taken, or to 

 have gained more than the weight of the food or drink taken; 

 the gain in such cases must have been due to the absorption of 

 water by the skin. Direct experiments however throw doubt on 

 these statements, for they shew that under ordinary circumstances 

 such a gain by the skin is slight, being apparently due to mere imbibi- 

 tion of water by the epidermis. It is uncertain whether substances in 

 aqueous solution can be absorbed by the skin when the epidermis 

 is intact, the evidence on this point being contradictory. In the 

 case of the sound human skin the balance of conflicting evidence 

 is in favour of the view that soluble non-volatile substances are 

 not absorbed, and that volatile substances such as iodine which 

 may be detected in the system after a bath containing them are 

 absorbed not by the skin but by the mucous membrance of the 

 respiratory organs, the substance making its way to the latter by 



