354 ESSENTIALS OF PHYSIOLOGY. 



mild character. Subsequent examination of the small pieces of pancreas 

 left in the animal reveals the fact that the islet tissue has lost its 

 granules, and has undergone other histological changes. It has yet to 

 be proved, however, that these changes are the cause and not a result 

 of the diabetes. 



The manner in which the internal secretion of the pancreas influences 

 carbohydrate metabolism is still obscure. Recent observations by 

 Starling show that the heart of a diabetic dog still possesses the power 

 of taking up sugar from the blood and utilising it, though probably to 

 a lesser degree than in the normal animal. Moreover, the respiratory 

 quotient in dogs, after extirpation of the pancreas, can be raised by 

 a liberal carbohydrate diet, an observation which suggests that their 

 tissues still possess some power of using carbohydrate. It is possible, 

 therefore, that the lessened capacity of the tissues to oxidise sugar may 

 not be the only factor concerned in experimental pancreatic diabetes. 



(6) Diabetes in man is a progressive disease characterised by 

 glycosuria, and due to a gradual failure of the tissues to assimilate 

 and oxidise dextrose, as is evidenced by the low respiratory quotient. 

 At first the sugar is derived only from carbohydrate, but eventually it 

 is also formed from protein, which is broken down to a larger extent in 

 order to supply to the tissues energy, which they cannot get from carbo- 

 hydrate. In the later stages of the disease the urine contains /?-oxy- 

 butyric acid and its products, often in large amount ; and the accumulation 

 of this acid in the body ultimately leads to poisoning of the tissue-cells, 

 which brings about coma and death. The blood contains an excess of 

 sugar, and after death the liver is almost free from glycogen ; in most 

 cases the pancreas shows signs of disease, but in others it appears normal. 

 Whether diabetes depends upon the deficiency or absence of an internal 

 secretion from the pancreas is not known, though from analogy with 

 experimental diabetes this seems very probable. 



SECTION IV. 



PROTEIN METABOLISM. 



The products of the digestion of protein are absorbed almost entirely 

 as ammo-acids, and there is no direct evidence that they are synthesised 

 into protein in the walls of the villi. On the contrary, there is no doubt 

 that the amino-acids enter the blood stream as such, and their presence 

 in the circulating blood can be demonstrated by the following means. 



An artery of an anaesthetised animal is connected with one end of a 

 series of tubes, the walls of which consist of a thin collodion membrane, 



