318 EXCRETION 



that each coil when unravelled measures about ^ of an inch (1.8 milli- 

 meter), the entire length of the secreting tubes is about 2^ miles (3} 

 kilometers). It must be remembered, however, that the length of the 

 secreting coil only is given and that the excretory ducts are not included. 



Mechanism of the Secretion of S^veat. The action of the skin as a 

 glandular organ is continuous and not intermittent ; but under ordinary 

 conditions the sweat is exhaled from the general surface in the form of 

 vapor. In regard to the mechanism of its separation from the blood, 

 nothing is to be said in addition to the general remarks on the subject 

 of secretion ; and it is probable that the epithelium of the secreting coils 

 is the active agent in the selection of the peculiar matters which enter 

 into its composition. There are no examples of the separation by 

 glandular organs of vapor from the blood, and the perspiration is se- 

 creted as a liquid, which becomes vaporous as it is discharged upon the 

 surface. 



The influence of the nervous system on the secretion of sweat is im- 

 portant. It is well known, for example, that an abundant production of 

 perspiration frequently is the result of mental emotions. Bernard has 

 shown that the nervous influence may be exerted through the sympa- 

 thetic system. He divided the sympathetic in the neck of a horse, 

 producing as a consequence an elevation in temperature and an increase 

 in the arterial pressure in the part supplied with branches of the nerve. 

 He found, also, that the skin of the part became covered with a copious 

 perspiration. On stimulating the divided extremity of the nerve, the 

 secretion of sweat was arrested. The local secretion of sweat after 

 division of the sympathetic in the neck of the horse was first observed 

 by Dupuy in 1816. 



The stimulation as well as the division of certain nerves induces local 

 secretion of sweat, but this nearly always is associated with dilatation of 

 the bloodvessels of the part ; still, sweat frequently is secreted when the 

 surface is pale and bloodless, showing that dilatation of the bloodvessels 

 is not an indispensable condition. The action of so-called vaso-dilator 

 nerves will be treated of in connection with the physiology of the 

 nervous system. In experiments on the cat, excito-secretory fibres have 

 been found to exist in the cerebro-spinal nerves going to the anterior 

 extremities. The fibres for the posterior extremities are in the sheath 

 of the sciatic nerve. In all instances the action of these nerves is direct 

 and not reflex. Experiments on the cat have been quite satisfactory, as 

 this animal sweats only on the soles of the feet and the secretion can 

 readily be observed. 



The so-called sweat-centres are in the lower part of the dorsal region 

 of the spinal cord, for the posterior extremities, and in the lower part of 



