430 ZOOLOGY FOR MEDICAL STUDENTS chap. 



is the single or paired preen-gland — ^the secretion of which is used for 

 oiling the feathers. In most birds this is situated on the dorsal surface 

 near the root of the tail : in the ordinary fowl and its allies on the side 

 of the head just behind the ear. 



The oldest known fossil birds (Archaeopteryx — Jurassic, Ichihyornis 

 and Hesperorhis — Cretaceous) possessed well -developed sharp conical 

 teeth, but in existing birds these have disappeared entirely, having been 

 replaced functionally by the horny beak. Up to the present time no 

 undoubted tooth vestiges have been found in bird embryos although 

 such may very possibly await discovery in some of the more primitive 

 birds. 



In correlation with their intensely active metabolism Birds possess 

 a very highly developed respiratory apparatus — the lungs attaining to 

 the highest grade of evolution known in the animal kingdom. In the 

 young developing bird there sprout out from the ventral wall of the 

 lung pocket-like outgrowths comparable with those of Chameleons. In 

 the bird, however, these outgrowths reach an immense size, becoming 

 great air-sacs with very delicate membranous walls which insinuate 

 themselves amongst the organs of the peritoneal cavity. The air-sacs 

 are five in number on each side and are named according to their position 

 in the body (i) cervical, (2) inter-clavicular (the right and left becoming 

 fused together), (3) anterior thoracic, (4) posterior thoracic and (5) 

 abdominal. 



Functionally the air-sacs are not directly respiratory, their walls being 

 non-vascular. They act as accessory organs to the respiratory process, 

 constituting a kind of bellows by which air is forced in and out of the 

 lung in the restricted physiological sense. The body-wall enclosing the 

 peritoneal cavity of the bird is traversed by a characteristic arrangement 

 of the skeleton. Dorsally is the vertebral column, its thoracic and 

 abdominal portions being rendered rigid by its constituent vertebrae 

 being fused together. Ventrally is the broad rigid sternum or breast- 

 bone. Laterally sternum and vertebral column are connected by the 

 highly-elastic >-shaped ribs with flat overlapping uncinate processes. 



The bird is able by contracting the muscles of its body-wall to draw 

 the sternum towards the vertebral column, bending the elastic ribs as it 

 does so. This lessens the volume of the peritoneal cavity, compresses 

 the air-sacs, and forces air from them outwards through the lung. When, 

 on the other hand, the muscles are relaxed, the elasticity of the ribs 

 forces the sternum in a direction away from the vertebral column, the 

 peritoneal cavity recovers its volume, and air is drawn back into the 

 air-sacs through the lung. The act of expiration is consequently in the 



