CLASS AVES 583 



The Digestive System. — Pigeons feed principally upon 

 vegetable food, such as seeds. The mouth cavity opens into the 

 oesophagus (Fig. 471, C), which enlarges into a crop (D) ; here the 

 food is macerated. The stomach consists of two parts, an an- 

 terior proventriculus (I) with thick glandular walls, which 

 secretes the gastric juice, and a thick muscular gizzard (K), 

 which grinds up the food with the aid of small pebbles swallowed 

 by the bird. The intestine forms a U-shaped loop, the duodenum 

 (M), which leads into the coiled small intestine, or ileum, and 

 finally passes into the rectum (R) at a point where two blind 

 pouches; the cceca (T), are given off. The alimentary canal leads 

 into the cloaca into which the urinary and genital ducts also open. 

 The cloaca opens to the outside by means of the anus (N). In 

 young birds a thick glandular pouch, the bursa Fabricii (0), 

 lies just above the cloaca. 



The two bile ducts (Hi, H2), one from each lobe of the liver 

 (L), discharge the bile into the duodenum. There is no gall- 

 bladder. The pancreas (P) pours its secretions into the duo- 

 denum through three ducts (Pi, P2, Pj). There is a spleen, 

 paired thyroids, adrenal bodies, and, in young pigeons, paired 

 thymus glands (see p. 492). 



The Circulatory System (Fig. 475). — The heart of a bird is 

 comparatively large. It is composed of two entirely separated 

 muscular ventricles (l.vn, r.vn) and two thin-walled auricles (l.au, 

 r.au). The right auricle (r.au) receives impure, venous blood 

 from the right precaval {r.prc), the left precaval (I. pre), and the 

 postcaval veins (ptc). This blood passes from the right auricle 

 into the right ventricle (r.vn), and is then pumped through 

 the pulmonary artery, which divides into right (r.p.a) and 

 left (l.p.a) pulmonary arteries, leading to the right and left 

 lungs respectively. 



The left auricle (Fig. 475, l.au) receives the blood which 

 returns, after being aerated in the lungs, through four large 

 pulmonary veins. It passes from the left auricle into the left 

 ventricle, and is then pumped through the right, aortic arch 



