AA'A TOMY OF BIRDS 309 



/. Splanchnology : the Digestive System. 



The Alimentary Canal, or digestive tract, is a tube which passes 

 through the body from mouth to anus, conveying food, the nutritious 

 qualities of Avhich are drawn off by the lacteals in transihi and 

 assimilated, the refuse being voided. This is digestion. The canal 

 is really a tube Avithin a tube, being contained in the cavity below 

 the bodies of the vertebnie, formed by the series of hcenvil archer. 

 Birds are very fast livers, their digestive operations, like the pro- 

 cesses of respiration and circulation, being very active and effectual; 

 they require proportionally greater quantities of food. The voracity 

 of the cormorant is proverbial, but it is probably not greater than 

 that of the ethereal nightingale. Birds as a class are omnivorous ; 

 many species are as nearly omnivorous as any animals can Avell be ; 

 but the majority are either vegetarian or flesh-feeding. Very many 

 birds feed upon fruits, hard or soft ; but even these, when in the 

 nest, are nourished for the most part upon the bodies of insects ; 

 and it may be truly said that the great majority of birds are 

 insectivorous. Birds seem to be the great controlling agency in the 

 economy of nature of the increase of insect life ; agriculture would 

 ])e difhcult if not im2)racticable without them, and their economic 

 value is simply incalculable. Insectivorous birds cannot be much 

 interfered with, without destroying one of the most important and 

 consequential of nature's many beautiful adjustments. The bird 

 cries perpetual " 6chec ! " to the insect. Even those birds which 

 are mainly flesh-eaters, as the hawks and owls, are similarly beneficial, 

 for the creatures they chiefly prey upon are the small rodents so 

 hateful to husbandry. The carrion-eaters contriltute largely to make 

 tropical regions habitable to man. Various tribes of birds feed 

 almost exclusively upon fish ; and these sometimes reach the dignity 

 of diplomatic and other political interests of mankind : nations have 

 gone to Avar over the dung of such birds, guano-beds being to some 

 of the South American poAvers a large item of their revenue. Chili 

 and Peru have been fighting latel}^, and the United States have been 

 Avrangling over the excrements of the alimentary canal of sea-birds. 

 This tube in general is shortest, simplest, and most direct in the 

 flesh- and fish-eaters, the nature of whose food assimilates already 

 more nearly to the substance of their bodies than does that of the 

 vegetarians. The tube is modified in different portions of its extent 

 for the prehension, retention, saturation, maceration, and comminu- 

 tion of food, and the mixture Avith it of other solvent fluids than 

 those secreted by the mucous membrane of the alimentary canal 

 itself. Hence arise the various modifications of its length, dilatation 

 here, contraction there ; the presence in its lining membrane of 

 numerous follicles ; and the annexation of various glandular organs. 



