OK, PLAIN TEACHING. 



109 



vorous birds. The food passes 

 through the oeso- 

 phagus (throat), 16, 

 into the crop, 17, 

 thence into a slight 

 enlargement of the 

 oesophagus, 19, 

 where it is mois- 

 tened by the gastric 

 Juice ; thence it 

 passes into the giz- 

 zard, 20, the thick 

 hard muscles of 

 which exercise a 

 grinding motion, 

 20 with pressure on 

 each other, like the 

 motion of two mill- 

 stones, and by these the grain, 

 or other vegetable matter, is re- 

 duced to a pulpy mass ; but as 

 this cannot be done without a 

 number of pebbles, or coarse par- 

 ticles of sand, are taken with the 

 food, the birds instinctively pick 

 them up, and they become em- 

 bedded in the coats of the gizzard. 

 Strange to say, it is this grinding 

 by the gizzard which affords 

 pleasurable sensation to birds, 

 and impels them to constantly 

 seek for food. In the duck tribe 

 the gizzard is enormously power- 

 ful, and will grind down hard 

 and sharp shells.* Carnivorous 

 birds have no gizzards. 



Birds are oviparous (egg pro- 

 ducing) ; their young are hatched 

 chiefly by the warmth of the 

 parent bird, or, in some instances, 

 chiefly by the heat of the sun. 

 The process of hatching is called 

 incubation. The eggs of different 

 species require various periods 



* The remaining processes of digestion are similar 

 to those which will he more fully described hereafter 

 In the description of other animals. 



for the process of development. 

 D omestic 

 fowls sit twen- 

 ty days. On 

 the twentieth 

 day the chick 

 being ma- 

 tured, 1, frees 

 itself from its 

 little prison 

 by breaking 

 the thin shell, 

 for which 

 purpose it is 

 provided with 

 a sharp pro- 

 jecting horny 

 point, placed 

 upon the 

 curved part of 

 the upper 

 mandible. 

 With this it 

 fractures the 

 shell, 2, and 

 escapes ; the horny point falls off 

 a day or two afterwards, being 

 no longer needed. 



Were birds to produce their young in any 

 other manner, their powers of flight would be 

 materially interfered with. As soon as an egg 

 becomes large enough to be cumbersome, it is 

 removed from the body. The shell, impervious 

 to air, protects the germ of life within, until 

 from two to twenty eggs have accumulated, and 

 then, although laid at different intervals, their 

 incubation commences together, and the young 

 ones are hatched at the same time. 



The yolk, 3, is absorbed, or 

 enclosed in the body of the chick, 

 and continues to nourish it for 

 some time after it has been 

 hatched, and while unable to 

 procure food for itself. In order 

 that the yolk may not be dis- 

 placed during the economy of 

 incubation, in which the egg is 

 repeatedly turned, it is moored 

 in position by numerous glutinous 



