SKIN. 159 



stances it is eliminated by the kidneys, appearing in the urine, constituting 

 the condition of glycosuria. 



Formation of Urea. The liver is now regarded by many physiologists 

 as the principal organ concerned in urea formation. The liver normally 

 contains a certain amount of urea; and if blood be passed through the ex- 

 cised liver of an animal which has been in full digestion when killed, a large 

 amount of urea is obtained. The clinical evidence proves that in destructive 

 diseases of the liver substance there is at once a falling-off in urea elimination. 

 Various drugs which stimulate liver action increase the amount of urea in 

 the urine. 



The antecedent of the urea, the substances out of which the liver cells 

 form urea, are for the most part the ammonium salts, the carbonate and 

 carbamate, which are brought to the liver by the blood of the portal vein. 

 These salts are formed largely in the intestinal wall out of the amino acids 

 that result from the digestion of proteins. It is also very probable that they 

 arise from the disintegration of proteins in other portions of the body. 



Influence of the Nerve System. The nerve system directly controls 

 the functional activity of the liver, and more especially its glycogenic function. 

 It was discovered by Bernard that puncture of the medulla oblongata is 

 followed by so enormous a production of sugar that it is at once excreted by 

 the kidneys, giving rise to diabetic or saccharine urine. This part of the 

 medulla is, however, the vaso-motor center for the blood-vessels of the liver. 

 Destruction of this center, or injury to the vaso-motor nerves emanating from 

 it in any part of their course, is followed at once by dilatation of the hepatic 

 blood-vessels, slowing of the blood-current, a profound disturbance of the 

 normal relation existing between the blood and liver-cells, and a production 

 of sugar. Many of the hepatic vaso-motor nerves may be traced down to the 

 cord as far as the lumbar region, while others leave the cord high up in the 

 neck and enter the cervical ganglia of the sympathetic, and so reach the 

 liver. Injury to the sympathetic ganglia is often followed by diabetes. 

 Peripheral stimulation of various nerves e. g., sciatic, pneumogastric, de- 

 pressor nerve, as well as the direct action of many drugs, impair or depress 

 the hepatic vaso-motor center and so give rise to diabetes. 



SKIN. 



The skin, the external investment of the body, is a most complex and im- 

 portant structure, serving 



1. As a protective covering. 



2. As an organ for tactile sensibility. 



3 . As an organ for the elimination of excrementitious matters. 



