210 VERTEBRATA. 



Class IV. AVES. (Birds.) 



Vertebrate, warm-blooded, oviparous animals, breathing by 

 lungs, and clothed with leathers. Heart with two auricles and 

 two ventricles. No corpus callosum. An amnion and allantois. 



The lower jaw is articulated to the skull by the intervention of 

 an os quadratum (representing the incus of mammals), and the 

 skull to the atlas by single occipital condyles. The cervical ver- 

 tebrae vary from nine to twenty-three. The coracoid process of 

 the scapula in mammals is a distinct bone in birds, and the two 

 clavicles are united to form the furculum [merrythought]. The 

 sternum, or breast-bone, is generally notched posteriorly [one or, 

 more rarely, two notches on each side] or perforated. In all 

 living birds the terminal tail-bones are anchylosed. The bones 

 are permeated with air derived from the bronchi, but in very 

 young birds they are filled with marrow. Air-cells are also 

 more or less interposed between the skin and the muscles. 



Most birds have an inferior larynx situated close to the bifur- 

 cation of the trachea ; sound is produced here arid is modified 

 above. In singing birds it is worked by five or six pairs of 

 muscles. The trachea is composed of bony rings, the bronchi of 

 half-rings situated on the outer side, the inner side being com- 

 pleted by a membrane. There is no epiglottis, or at least it is 

 only rudimentary ; the papillae at the base of the tongue, directed 

 backwards, prevent the food from entering the trachea. The 

 tongue is more an organ of touch than of taste. The oesophagus, 

 always very dilatable, terminates in the " proventriculus," which 

 is lined with the glands secreting the gastric juice. Close to the 

 proventriculus is the stomach, thin and membranous in carni- 

 vorous, muscular in granivorous birds. The intestine is from three 

 to nine times the length of the body ; it is longest in gramini- 

 vorous birds. 



In many birds, such as those that gorge themselves at uncertain 

 intervals, and those which, living exclusively on vegetable food, 

 require a temporary store, the oesophagus dilates to form a 

 " crop" [ingluvies]. From the crop, or from the stomach, many 

 birds, such as kingfishers, parrots, pigeons, &c., have the power 

 of producing their partially digested food for the support of 

 the female during, and the young after, incubation ; in the latter 

 and a few others the crop secretes a peculiar milky fluid. Some 

 birds also have the power of casting up the indigestible matter 

 which they iiave swallowed. 



