BIRDS I47 



p. 6), with a larynx (superior larynx). Since, however, no vocal cords are 

 present in this organ, it cannot be used for the production of sound. 

 The real vocal apparatus of birds is formed by a dilatation of the lower 

 end of the trachea, the so-called lotcer larynx or syrinx, in which vocal 

 cords are present, and which in the song-birds displays a specially high 

 degree of development. 



8. Organs of Digestion. — As has been seen above, flight is the most 

 difficult form of motion, and entails a very great expenditure of force. 

 This in its turn conditions a large consumption of food (see Part I., p. 8), 

 and a bird spends almost the whole of the time when he is not asleep in 

 the eager search for nourishment. 



In the absence of teeth (see Section 5), the task of crushing the food 

 devolves upon the stomach. This organ varies in structure according to 

 the nature of the food, whether this consists of hard or soft substance 

 (grain and hard-shelled insects, or the flesh of vertebrates, snails, soft- 

 bodied insects, etc.) . In the former case the stomach is highly muscular, 

 forming a gizzard, which crushes and grinds like the stones of a mill (see 

 pigeon) ; in the latter case it consists of a membranous sac (see the buz- 

 zard), in which the flesh food is dissolved by the strongly acid gastric 

 juices as in a chemical retort {retort-stomach). For remarks on the tongue, 

 salivary glands, oesophagus and crop, see under the treatment of the sepa- 

 rate species. Birds have no urinary bladder. (This effects a saving in 

 body-weight.) The ureters, the intestine and the oviducts terminate in a 

 common receptacle, the cloaca (see also duck-bill). The urine of birds is 

 white, and is discharged simultaneously with the faeces. 



9. The Heart is constructed on the same plan as in mammals (see 

 Parti., p. 16). 



10. Feathers. — Increased consumption of food has the effect of in- 

 creasing the heat of the animal body, which is produced by its combustion 

 (see Part I., pp. 7, 8). Birds accordingly are warm-blooded creatines. 

 The temperature of the body must, however, be kept at the same height, 

 in order to prevent any interruption or cessation of the internal activities 

 of the body. Birds therefore require (like mammals, see Part I., p. 11) 

 special means for conserving the body-heat. These are provided primarily 

 by the numerous warm air-spaces among their thick, warm plumage. The 

 air in the air-sacs likewise acts as a heat-conserving agent. (Birds while 

 roosting, or in cold weather, vigorously fill their air-sacs with air, so 

 that the feathers come to stand on end, as may be easily observed in 

 sparrows or canaries. In this way the organs of the body become, as it 

 were, enveloped in air, and the spaces between the feathers still further 

 enlarged.) Pat is met with in birds only exceptionally (swimming birds) 

 as a means for conserving body-heat, since it would load the body too 



