EXCRETION 419 



mechanism ; and it is certain that dyspnoea 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 perspira- 

 tion broke out several times on the upper part of the body, while the 

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



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 

 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 con- 

 trast 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 



