436 THE ELIMINATION OF WASTE PRODUCTS. 



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 lead to speak of sweat centres, analogous to the vasomotor 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 

 in the medulla oblongata or whether such centres are more generally dis- 

 tributed 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 manher ; 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 pilo- 

 carpine, 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 pilocarpine apparently has as well some slight action on 

 the nerve centres) and the antagonistic action of atropine is similarly local. 

 Picrotoxine and strychnine appear to produce their sweating action chiefly 

 if not exclusively by acting on the central nervous system, while nicotine 

 seems to act both centrally and peripherally. 



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

 spinal cord by the roots of the twelfth thoracic to the third lumbar nerve 

 inclusive, pass along the rami communieantes 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. The sweat-nerves for the 

 fore-foot leave the spinal cord by the roots of the fourth to the ninth or 

 tenth, chiefly in the sixth, seventh, and eighth dorsal nerve, inclusive, pass 

 into the thoracic sympathetic, thence into the ganglion stellatum, and so 

 join the brachial plexus by the fine branches passing from the ganglion to 

 the spinal nerves. The course to the fore-foot is finally along the median 

 and ulnar nerves respectively. In the horse the sweat-fibres for the side of 

 the face and in the pig those for the snout appear to run in branches of the 

 fifth nerve and not in the facial ; in the latter animal at least some of these 

 fibres reach the fifth nerve from the cervical sympathetic, but apparently 

 not all. 



371. The fact mentioned above that in the horse, after section of the 

 cervical sympathetic nerve on one side of the neck, profuse sweating is apt 

 to break out on that side of the face, has suggested the idea that this nerve 

 conveys inhibitory impulses to the sweat-glands of the head and face, and 

 that when it is divided the sweat fibres running in the fifth nerve, having 

 nothing to counteract them, set up sweating. But it is probably sufficient 

 in this case to suppose that the glands predisposed to activity by the higher 

 temperature brought about by the section of the sympathetic dilating the 

 bloodvessels, are more easily excited by any stimulus working upon them 

 through the fifth nerve. And though the idea of a double nervous mechan- 

 ism, augmenting and inhibitory, governing the activity of the sweat-glands, 

 is a tempting one, there are at present no satisfactory reasons for adopting it. 



