5 I 4 EXCRETION 



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 ranii 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 they form connections 

 with ganglion cells ; then, as non-medullated fibres, run along the grey 

 rami, and are distributed to the foot in the sciatic. 



The evidence of the direct secretory action of nerves on the sweat- 

 glands is singularly striking and complete, in contrast to what we 

 know of the kidney. In the latter, blood-flow is the important 

 factor; increased blood-flow entails increased secretion. In the 

 former, the nervous impulse to secretion is the spring which sets 

 the machinery in motion; vascular dilatation aids secretion, but 

 does not generally cause it. It would, however, be easy to lay too 

 much stress on this distinction, for in the horse the mere dilatation 

 of the bloodvessels of the head after section of the cervical sympa- 

 thetic has been found to be accompanied by increased secretion of 

 sweat, and urinary secretion can certainly be affected by the direct 

 action of various substances on the secretory mechanism, indepen- 

 dently of vascular changes. But the broad difference stands out 

 clearly enough, and the reason of it lies in the essentially different 

 purpose of the two secretions. The water of the urine is in the 

 main a vehicle for the removal of its solids; the solids of the sweat 

 are accidental impurities, so to speak, in the water. The kidney 

 eliminates substances which it is vital to the organism to get rid 

 of; the sweat-glands pour out water, not because it is in itself 

 hurtful, not because it cannot pass out by other channels, but 

 because the evaporation of water is one of the most important 

 means by which the temperature of the body is controlled. In 

 short, urine is a true excretion, sweat a heat-regulating secretion. 

 No hurtful effects are produced when elimination by the skin is 

 entirely prevented by varnishing it, provided that the increased 

 loss of heat is compensated. A rabbit with a varnished skin dies 



