436 A HISTORY OF BIRDS 



Thus, then, there is a definite relation between the length 

 and thickness of the gut and the nature of the food. In Grani- 

 vorous birds the gut is of great length, and this may be traced 

 to the fact that the food is difficult to digest and is mixed with 

 a large amount of indigestible material, so that much time and 

 a large surface for the action of the digestive juices are neces- 

 sary for the conversion of the ingested material into food. 

 Birds which feed on fish similarly require a gut of great length, 

 and in these birds the walls of the intestines are much 

 thickened, probably as a protection against injury by sharp 

 bones. Though fish is popularly regarded as a most nutritious 

 diet, it must be remembered that birds have to eat their cap- 

 tures raw, and it has been shown that in uncooked fish no less 

 than 70 per cent, is wasted. 



Frugivorous birds have short thin-walled intestines. Here, 

 as Dr. Mitchell points out, the nutritive substances are in forms 

 which render them capable of rapid and fairly complete ab- 

 sorption, and the organic salts present stimulate osmosis, or 

 the passage of the digested matter through the walls of the 

 intestine into the blood. The ease of absorption makes a 

 relatively large surface unnecessary, and the large calibre of the 

 gut not only diminishes the outflow from the blood caused by 

 the presence of organic salts, but it decreases the danger of vio- 

 lent purging. 



Such are the general principles, but the variety displayed 

 by the coilings of these intestines within the body cavity is 

 infinite. The fact that unrelated types often resemble one 

 another, while others present sudden and remarkable variations, 

 makes the study of the intestinal tract one of great difficulty ; 

 at the same time it furnishes most valuable data to those who 

 desire to investigate the problem of the evolution of the various 

 systems of the body apart from the evolution of the organism 

 as a whole. 



The diverticulum, which represents the last vestige of the 

 yolk-sac, is in some birds completely lost, in others it remains 

 as a vestige throughout life, while yet again in others it has 

 become transformed into a gland and takes on a new function, 

 apparently intimately correlated with the development of the 

 caeca. 



The caeca are blind diverticula of the intestine placed at 



