XIII 



PHYLUM CHORDATA 381 



usually heterocoelous, but may be proccelous or amphicoelous. The 

 sacral vertebrae are fused with the lumbar and with more or fewdr 

 of the posterior thoracic and anterior caudal to form a syn-sacrum 

 for the support of the ilia. The posterior caudal vertebrae are 

 usually fused to form a pygostyle around which the tail-quills are 

 arranged in a semicircle. The bones of the skull undergo early 

 ankylosis. There is a single, rounded, occipital condyle ; the united 

 premaxillaB 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 sy no vial 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 ischiadic 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 cceca. 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 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 pccten. The auditory organ has a large 



