396 ZOOLOGY 



SECT. 



united premaxillse form nearly the whole of the upper jaw ; and 

 the lower jaw is composed originally of five or six bones in each 

 ramus, and is supported by a freely articulated quadrate. The 

 vertebral ribs are double-headed, provided with bony uncinates, 

 and articulate with the bony sternal ribs by synovial joints. The 

 sternum is broad, and is typically produced into a longitudinal 

 ventral keel, having a separate centre of ossification. The coracoid 

 is usually more or less pillar-like, the scapula is sabre-shaped, 

 and the clavicles and interclavicle unite to form a furcula. Except 

 in one extinct species the distal carpals and the metacarpals are 

 united to form a carpo-metacarpus. There are usually only three 

 digits in the wing, which probably represent the first, second, and 

 third of the typical hand. The ilium is of great size, having large 

 pre- and post-acetabular portions. The acetabulum is perforated 

 in the dry bone. The pubis and ischium are directed backwards 

 and, except in one case of each, there is neither pubic nor ischiatic 

 symphysis. The head of the femur is at right angles to the shaft. 

 The proximal tarsals are fused with the tibia to form a tibio-tarsus ; 

 the fibula is much reduced. The distal tarsals are fused with the 

 second, third, and fourth metatarsals to form a tarso-metatarsus ; 

 the first metatarsal is free. The fifth digit of the typical foot is 

 absent. 



In all tertiary and recent Birds teeth are absent. The gullet is 

 frequently dilated into a crop and the stomach is usually divided 

 into proventriculus and gizzard. The junction between the large 

 and small intestines is marked by a pair of caeca. The lungs are 

 spongy and non-distensible. The bronchi give off branches which 

 open on the surface of the lung into thin-walled air-sacs, and these 

 in their turn usually communicate with pneumatic cavities in more 

 or fewer of the bones. The voice is produced in a syrinx situated 

 at or near the junction of the trachea with the bronchi. The heart 

 is four-chambered, the right auriculo-ventricular valve is muscular, 

 and the right aortic arch alone is present in the adult. The renal 

 portal system is vestigial. The red blood-corpuscles are oval and 

 nucleated. The temperature of the blood is high (about 38 C.). 

 The optic lobes are displaced laterally owing to the meeting of the 

 large cerebral hemispheres and cerebellum. The lumbar region of 

 the spinal cord has a sinus rhomboidalis. The olfactory organ is 

 usually poorly developed. The eye is usually large, and has 

 sclerotic plates and a pecten. The auditory organ has a large 

 curved cochlea, The kidney is three-lobed, and is developed from 

 the metanephros, the mesonephros undergoing atrophy. There is 

 no urinary bladder. The ovary and oviduct of the right side are 

 more or less completely atrophied. 



Birds are all oviparous, and the large ovum, containing much 

 food-yolk, becomes invested with albumen, a double shell-mem- 

 brane, and a calcareous shell, in its passage down the oviduct. The 



