CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 735 



lation of the sciatic in the cat 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 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 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 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 spinal bulb 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 dyspnoaa 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 antagonistic 

 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. 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 corresponding to 

 these ganglia and so the sciatic nerve. Along their course the 



