vi FORM AND FUNCTION 67 



work, the process of digestion is complete, and the 

 food goes to build up the living animal. The wonder- 

 ful process mentioned above is in full swing here ; 

 the chyle, that part of the food which consists of 

 emulsified fats, passes into the lacteals (to be described 

 later on) through the villi, small creases in the coat of 

 the intestine, the rest into the blood-vessels, and so, in 

 either case, on to the heart. All refuse is carried into 

 the large intestine, any return from which is prevented 

 by a valve. 



Before leaving the subject of digestion, I wish 

 to show how important an organ the liver is to birds. 

 For purposes of flight their weight is reduced as much 

 as possible, but in some good flyers the liver is ex- 

 traordinarily heavy. In the Tern it is ^ of that of 

 the whole bird, in the Swallow T V, in Yanellus 

 Cristatus, a kind of Lapwing, ^, in the Smew Jy. In 

 all these it forms a far larger fraction of the whole 

 than in man. And how account for the great differ- 

 ences ? The Smew is mainly a fish-eater and also the 

 Tern. But in the fish-eating Heron the liver is said 

 to be remarkably small. In the common Fowl it 

 contributes rather less than Y \ of the total weight, 

 suggesting that seed-eaters depend comparatively- 

 little on the liver and more upon the gizzard. But 

 in corn-eating pigeons we find both large gizzards and 

 large livers. And the liver of the flesh-eating gizzard- 

 less Kestrel is lighter than that of the Fowl, only T .V, in 

 fact, of his total weight, while in the Tawny Owl it is 

 less than J^.. These facts are very perplexing. Dr. 

 Gadow in quoting them remarks that they are un- 

 trustworthy, since thev must be affected by the con- 



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