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, 1. The food 
passes down the gullet and lodges in the crop, ¢, 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 110° or 112°. This is due to the fact 
that the birds are extremely active, and that the blood is 
not only aérated 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, Larus, 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. 
