AVES 267 



principal types of feathers are to be distinguished: a, contour feathers, 

 which include the flight feathers; b, down feathers, possessing a soft 

 shaft and a vane without barbules; c, Jiloplumes, with slender, hair- 

 hke shaft and few or no barbs. Feathers are arranged in tracts, 

 called pteryla, with naked spaces between, called apteria. Moulting 

 of feathers occurs periodically, old feathers being dropped and new 

 ones, sometimes of different color, growing out of the old follicles. 



The Skin is dry and practically without glands. The only skin 

 gland is a single oil gland on the tail; even this is absent in some 

 species. 



The Skeleton (Fig. 144). — Most of the skeletal peculiarities have 

 been already discussed. The stemiun is keeled except ostriches, etc.; 

 ribs have uncinate processes, except Screamers; skull is rounded, has 

 large orbits and the facial bones are extended out upon the beak; 

 quadrate is movable and articulates with the squamosal; a single oc- 

 cipital condyle; no teeth, except extinct forms; cervical vertebrae 

 have saddle-shaped articular surfaces, giving the neck great flexi- 

 bility and rendering the beak an unusually versatile implement; trunk 

 vertebrae mostly fused; three or four free caudal vertebrae with term- 

 inal pygostyle: two cervical and three to nine thoracic ribs, the latter 

 attached to the sternum; pectoral girdle consists of paired blade-like 

 scapulae, paired coracoids, that are united to the sternum, and free 

 clavicles, fused in the middle to make the "wish-bone"; the pelvic 

 girdle is a solid bone, consisting of the fused ischia, ilia, and pubes, 

 and the pelvis is firmly fused with the sacral vertebrae; the wing skel- 

 eton has been sufficiently described; the leg skeleton consists of a 

 Isbige femur, a slender fibula and the long, stout tibio-tarsus, composed 

 of the fused tibia and proximal tarsal bones; the ankle joint is be- 

 tween the tibio-tarsus and the tarso-metatarsus; foot has four digits, 

 with hallux usually directed backward. 



Digestive System. — Mouth hard and narrow; tongue hard and 

 often of great functional value; oesophagus with enlargement, called 

 the crop; stomach with proventriculus that secretes gastric juice, and 

 a muscular gizzard or gastric mill; intestine U-shaped, composed of 

 duodenum, ileum and rectum; between ileum and rectum are two cceca; 

 rectum opens into a cloaca. There are two bile ducts but no gall- 

 bladder; a pancreas empties into the duodenum. 



Circulatory System. — The heart is large and four chambered; right 

 auricle receives venous blood, left receives blood from the lungs; the 



