CHAP, in.] ELIMINATION OK WASTE PRODUCTS. 701 



secretion in a limb exposed to a temperature of 35 or somewhat 

 above, than in one which lias been exposed to a distinctly lower 

 temperature, and in a limb which has been placed in ice-cold 

 water hardly any secretion at all can be gained; but apparently 

 mere rise of temperature without nerve-stimulation will not giv<j 

 rise to a secretory activity of the glands. The sweating caused 

 by a dyspnceic condition of blood, and such appears to be the 

 sweat of the death agony, is similarly brought about by the 

 agency of the central nervous system. When an animal with the 

 sciatic nerve divided on one side is made dyspnceic, no sweat 

 appears in the hind limb of that side, though abundance is seen in 

 the other feet. 



Sweating may be brought about as a retiex act. Thus when 

 the central stump of the divided sciatic is stimulated sweating is 

 induced in the other limbs, and in ourselves the introduction 

 of pungent substances into the mouth will frequently give rise to a 

 copious perspiration over the side of the face. We are thus led to> 

 speak of sweat centres, analogous to the vaso-motor centres, as 

 existing in the central nervous system ; and as in the case of vaso- 

 motor centres, a dispute has arisen as to whether there is a 

 dominant sweat centre in the medulla oblongata or whether such 

 centres are more generally distributed over the whole of the spinal 

 cord. 



It does not at present appear certain whether the sweating 

 caused by heat is carried out by direct action of the heated blood 

 on the sweat centres, or by the higher temperature stimulating 

 the skin and so sending up afferent impulses which produce the 

 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 apparently has 

 as well some slight action on the nerve centres), and the antagon- 

 istic action of atropin is similarly local. Picrotoxin and strychnia 

 appear to produce their sweating action chiefly if not exclusively 

 by acting on the central nervous system, while nicotin seems to 

 act both centrally and peripherally. 



442. The sweat-fibres for the hind foot (in the cat) appear^ 

 to leave the spinal cord by the roots of the lower dorsal and upper 

 lumbar nerves, pass along the rami communicantes to the abdominal 

 sympathetic, and thus reach the sciatic nerve. They thus follow 

 very much the course of the vaso-constrictor fibres of the lower 

 limb ; but the particular spinal nerves by which the sweat-fibres 

 issue from the cord have not yet been definitely settled, and 

 possibly they are in the dog and cat the last two or the last three 

 dorsal and first two or four lumbar nerves. Similarly the sweat- 

 nerves for the fore-foot leave the spinal cord by the roots of 



