748 



PHYLUM CHORDATA : CLASS AVES — BIRDS 



food is ground. The walls are very muscular, the fibres 

 radiating from two tendinous discs ; the internal surface is 

 lined by a hard, horny epithelium ; and within the cavity 

 are small stones which the bird has swallowed. In hawks 

 and fish-eating birds the gizzard region is, naturally enough, 

 soft. The pyloric opening, from the gizzard into the 

 duodenum, is very near the cardiac opening from the 

 proventriculus into the gizzard. 



In the fold of the long duodenum lies the pancreas with 



three ducts, whose number points to 

 the triple origin of the pancreatic 

 outgrowth in the embryo. Into the 

 same region open two bile-ducts from 

 the two-lobed liver, which is without 

 a gall-bladder in the common pigeon, 

 though this is present in some birds, 

 and even in some species of pigeon. 



Fig. 448. — Diagram- 

 matic section of clo- 

 aca of male bird. — 

 After Gadow. 



cd.. Upper region of cloaca 

 into which rectum opens ; 

 ud., median region into 

 which ureter (m.) and vas 

 deferens {v.d.) open from 

 each side ; pd., posterior 

 region into which bursa 

 Fabricii (B.F.) opens. 



The small intestine is long ; the large 

 intestine very short — not more than a rectum 

 two inches in length. At the junction of 

 the small and the large intestine there are 

 two short caeca. In some birds, e.g. the fowl, 

 these are of considerable length ; in the 

 ostrich they are very long ; there are three 

 in many ducks and birds of prey ; there is 

 only one in some fish-eating birds ; in horn- 

 bills, parakeets, etc., they are absent. 



The cloaca has three divisions (see Fig. 

 448) — an upper part into which the rectum 

 opens, a median part into which the ureters 

 and the genital ducts open, and a posterior region (proctodaeum), 

 opening into which from the dorsal surface is a sac of obscure 

 function, the bursa Fabricii, which usually disappears during adoles- 

 cence. It is at first a blood-forming organ, but often becomes a mass 

 of fibrous connective tissue. 



Vascular system. — The relatively large four-chambered 

 heart, the complete separation of arterial and venous blood, 

 the single aortic arch bending over to the right side, and 

 the hot blood (about 38' C, 100° F.), are important 

 characteristics. The heart-beats are more rapid in birds 

 than in other Vertebrates, being about 120 per minute 

 when the bird is at rest, and far more when it is flying. 



The impure blood returned by the venge cav^ to the 



