334 SYSTEMATIC ZOOLOGY 





without teeth in all living birds, although some extinct birds had 

 a good supply of them. The tongue is well developed, conform- 

 ing in shape to the shape of the beak, and usually has a firm 

 covering. The form and size of the beak vary enormously ac- 

 cording to the nature of the bird's food. The oesophagus leads 

 from the mouth to the stomach, and sometimes has a saclike 

 enlargement on its ventral wall, the crop, which serves for the 

 storage of food, and in some cases acts upon it by glandular 

 secretions. The stomach generally consists of two parts : the 

 anterior, glandular stomach, and just posterior to this the mus- 

 cular stomach, or gizzard, where the food is ground up. From 

 the gizzard a long, coiled intestine passes to the cloaca, which 

 opens on the ventral side of the body at the root of the tail. 

 Opening into the intestine near the stomach is a gland, the 

 pancreas, whose secretion assists in digestion, and the secretion 

 of bile from the liver enters the intestine in the same region. 

 Near the intestine is a small, red body, the spleen, whose func- 

 tion is unknown. 



The air passes into the nostrils to the back of the mouth, 

 and then into the windpipe, or trachea, which divides at its 

 posterior end into two bronchi, going to the lungs. The open- 

 ing into the trachea is called the glottis, and this is succeeded 

 by the larynx, which in other vertebrates is the organ of voice 

 or sound ; but in the birds all sound is produced by a second 

 larynx, which lies at the opposite end of the trachea, at the 

 point where it divides into the two bronchi ; this second larynx 

 is technically called the syrinx. The lungs are thin-walled 

 sacs which connect with a number of very thin air sacs placed 

 in various parts of the bodv. and these in turn communicate 

 with the air spaces in the bones already referred to. In some 

 birds there are even air spaces in the skin as well. 



The heart is a relatively large, muscular organ, lying dorsal to 

 the sternum, and consists of four chambers, two auricles with 

 thin walls, and two ventricles with thick walls. The blood is col- 

 lected from the various parts of the body by veins, and pas 

 into the right auricle, from which it goes to the right ventricle, 

 and then to the lungs ; after being aerated it flows to the left 

 auricle, thence to the left ventricle, and then into the arteries, 

 which carry it to all parts of the body. The excretory organs 



