EXCRETION 47 5 



increases the flow of sweat by direct stimulation of the endings 

 of the secretory nerves in the glands. 



That the sweating caused by a high external temperature is 

 normally brought about by nervous influence, and not by direct 

 action on the secreting cells, is shown by the following experi- 

 ments. One sciatic nerve is divided in a cat, and the animal 

 put into a hot-air chamber. No sweat appears on the foot 

 whose nerve has been cut, but the other feet are bathed in 

 perspiration. Similarly, a venous condition of the blood (in 

 asphyxia) causes sweating in the feet whose nerves have not 

 been divided, but none in the other foot ; and stimulation of the 

 central end of the cut sciatic has the same effect. All this points 

 to the existence of a reflex mechanism ; and it is certain that 

 asphyxia acts by direct stimulation of the centre or centres. 

 The vaso-motor centre is at the same time stimulated, and the 

 bloodvessels constricted, as in the cold sweat of the death 

 agony. Fear may also cause a cold sweat, impulses passing 

 from the cerebral cortex to the vaso-motor and sweat centres. 



It is probable that a general sweat-centre exists in the medulla 

 oblongata, but its position has not been exactly determined nor even 

 its existence definitely proved. On the other hand, it is known that 

 in the cat there are at least two spinal centres, one for the fore-limbs 

 in the lower part of the cervical cord, and another for the hind-limbs 

 where the dorsal portion of the cord passes into the lumbar. That 

 this latter centre does not exist or is comparatively inactive in man 

 is indicated by the following case: A man fell from a window and 

 fractured his backbone at the fifth dorsal vertebra. The lower half 

 of the body was paralyzed for a time, but recovered. Ultimately, 

 however, the paralysis returned ; and shortly before his death 

 (twenty-one years after the accident) it was noticed that a copious 

 perspiration broke out several times on the upper part of the body, 

 while the lower portion remained perfectly dry. If there is any 

 functional spinal centre in man, it appears to lie above the fifth spinal 

 segment. For it was seen in a professional diver who fractured his 

 neck at that level, and lived three months after the accident, that 

 sweat frequently appeared on parts of the body above the lesion, but 

 never below. At the autopsy the whole thickness of the cord, 

 except perhaps a small portion of the anterior columns, was found 

 destroyed. Of course, it may be that in man the spinal centres, 

 although normally active, lose their function for a long time after 

 such severe injuries to the cord owing to the condition known as 

 shock. 



The secretory fibres for the fore-limbs (in the cat) leave the cord 

 in the anterior roots of the fourth to ninth tHoracic nerves. They 

 pass by white rami communicantes to the sympathetic chain, in which 

 they reach the ganglion stellatum, where they are all connected with 

 nerve-cells. Then, as non-medullated fibres, they gain the brachial 

 plexus by the grey rami, and run in the median and ulnar to the 

 pads of the feet. The fibres for the hind-limbs leave the cord in the 

 anterior roots of the twelfth thoracic to the third or fourth lumbar 

 nerves ; pass by the white rami to the sympathetic ganglia, in which 



