228 BACKBONED ANIMALS. 



Digestion. As the birds have no teeth, they either 

 swallow their food entire or tear it with the bill or claws. 

 The digestive organs are shown in Fig. 269, i. The food 

 passes down the gullet and lodges in the crop, c, that is 

 easily felt in chickens that have gorged themselves with 

 corn. From here it passes to the true stomach just be- 

 low, and is brought in contact with a secretion called 

 gastric juice. From here it passes to the gizzard, g, that 

 to all intents and purposes is an internal set of teeth or 

 grinders, being a muscular sac with a hard, horny lining 

 in which the grain or other food is completely ground to 

 a pulp. To assist in this operation chickens and other 

 grain-eating birds swallow gravel and pebbles. In the 

 flesh-eaters, as the eagles, the coat of the gizzard is not so 

 thick. The experiment has been tried of feeding gulls on 

 grain,* and it was found that the gizzard assumed the ap- 

 pearance and functions of that of true grain-eaters. When 

 the food is thoroughly ground, that which is not absorbed 

 as fuel for the system enters the small intestine and is 

 finally rejected. 



Circulation. In the birds we meet for the first time a 

 warm-blooded animal, the mean temperature of the blood, 

 which is red, being no or 112. This is due to the fact 

 that the birds are extremely active, and that the blood is 

 not only aerated in the lungs, but in the air-sacs of the 

 various parts of the body. Again, the feathers are poor 

 conductors of heat, and tend to keep up the body tempera- 

 ture. The heart is now four-chambered, composed of two 

 auricles and two ventricles. In circulating, the venous 

 blood enters the right auricle, flowing from here to the 

 right ventricle, from which it passes through the pul- 

 monic artery to the lungs. Here it changes into arterial 

 blood, passes to the left auricle, then to the left ventricle, 



* A gull, Lartts t at the Shetland Islands, lives on grain in the sum- 

 mer and fish in the winter, a habit that must cause a yearly physical 

 change. 



