CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 557 



produces a much more abundant secretion in a limb exposed to 

 a temperature of 35 or somewhat above, than in one which has 

 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 give rise to a secretory 

 activity of the glands. The sweating caused by a dyspnoeic 

 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 dyspnoeic, 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 reflex 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 cen- 

 tres, 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 pilo- 

 carpin apparently has as well some slight action on the nerve 

 centres), and the antagonistic action of atropin is similarly 

 local. Picrotoxin and strychnia appear to produce their sweat- 

 ing action chiefly if not exclusively by acting on the central 

 nervous system, while nicotin seems to act both centrally and 

 peripherally. 



354. In the cat (in which animal the matter has been most 

 studied), the sweat fibres for the hind-foot leave the spinal 

 cord by the anterior roots of the first and second lumbar nerves, 

 but also to a less extent by the two thoracic nerves above these 

 and the third lumbar nerve below. Passing to the sympathetic 

 chain, and running in it for a certain distance, they leave that 

 chain by the grey rami of the sixth and seventh lumbar and 

 first and second sacral ganglia, thus reaching the spinal nerves 



