528 Comparative Morphology of Chordates 



appears completely, leaving the heart connected with the dorsal aorta 

 by only the right half of the aortic arch, connected with the left 

 ventricle (Fig. 1SF). Therefore all parts of the adult arterial system 

 carry "pure" oxygenated blood. 



The rate of heartbeat in birds is incredibly rapid. The "basal" 

 rates in an English sparrow, a canary, and a hummingbird have been 

 reported as being, respectively, about 350, 500, and 600 beats per 

 minute (in contrast to about 75 in man). 



Digestive Organs 



The digestive organs of birds exhibit some highly distinctive fea- 

 tures. The length of the esophagus necessarily varies greatly according 

 to the length of the neck. In the large majority of birds, the posterior 

 region of the esophagus is more or less dilated to form a chamber, the 

 crop (ingluvies), which serves for temporary storage of the food 

 before it is passed on for digestion. The possession and degree of de- 

 velopment of the crop is closely correlated with the feeding habits of 

 the bird. In omnivorous birds and those whose chief food is insects or 

 fruit, the crop is usually small or lacking. It is strongly developed in 

 birds whose main food is grain or seeds. It is especially prominent in 

 such large birds of prey as the vulture and eagle. It is well developed 

 in the domestic fowl, pigeon, and duck but lacking in the goose, swan, 

 and most waders. In general, the crop is small or absent in birds whose 

 food is easily digested and taken in small quantities at frequent inter- 

 vals, but more strongly developed when the food is difficult to digest 

 (e.g., grain) or when it is taken in enormous quantity at a single meal 

 with long and irregular intervals between meals, as in birds of prey. 

 Usually the crop serves only for storage, but in grain-eaters its lining 

 secretes a fluid which softens the food and prepares it for the action of 

 digestive secretions. In pigeons the crop during the breeding season 

 produces "pigeon's milk," which is regurgitated and fed to the young 

 birds. It resembles mammalian milk in appearance, but the two are 

 unlike chemically. 



The region between the esophagus and the intestine is more or less 

 definitely divided into two chambers (Fig. 412). The anterior one 

 (proventriculus) is usually smaller than the other, the muscular 

 layer of its wall is thin, and its epithelial lining is thick and closely 

 beset with glands which secrete the characteristic digestive fluids of a 

 stomach. The posterior chamber (gizzard), when strongly developed, 

 is much the larger and has an enormously thickened muscular layer 

 differentiated into a massive and powerful muscle on each of two 

 opposite sides of the organ. The internal epithelium is highly glandular. 

 The secretion, however, has no digestive function but is a substance 



