THE SKIN AND SKELETON. 
145 
the roof of the nose. 8. The two maxilla e; that part of 
the upper jaw in which the canines, premolars, and molars 
are lodged. 9. The two premaxilla, in which the upper 
incisors are situated. 10. The two palatines, which, with 
the maxillary hones, form the roof of the mouth. There 
are two appendages to the skull: the mandible , or lower 
jaw, w T hose condyles, or rounded extremities, fit into a 
cavity (the glenoid) in the temporal bone; and the hyoid 
bone, situated at the root of the tongue. 
The simplest form of the skull is a cartilaginous box, 
as in Sharks, enclosing the brain and supporting the car¬ 
tilaginous jaws and gill arches. In higher Fishes this box 
is overlaid with bony plates and partly ossified. In Frogs 
the skull is mainly bony, although a good deal of the car¬ 
tilage remains inside the bones. In higher Vertebrates the 
cartilage never makes an entire box, and early disappears. 
The cervical vertebra , or bones of the neck, are peculiar 
in having an orifice on each side of the centrum for the 
passage of an artery. The first, called atlas, because it 
supports the head, has no centrum, and turns on the sec¬ 
ond, called axis, around a blunt process, called the odon¬ 
toid. The centra are usually wider than deep, and the 
neural spines very short, except on the last one. The 
number of cervical vertebrae ranges from 1 in the Frog 
to 25 in the Swan. 
The dorsal vertebra are such as bear ribs, which, uniting 
with the breast-bone, or sternum , form a bony arch over 
the heart and lungs, called the thorax. The sternum may 
be wanting, as in Fishes and Snakes, or greatly developed, 
as in Birds. When present, the first vertebra whose ribs 
are connected with it is the first dorsal. The neural spines 
of the dorsal series are generally long, pointing backward. 
The lumbar vertebra are the massive vertebrae lying in 
the loins between the dorsals and the hip-bones. 
The sacral vertebra lie between the hip-bones, and are 
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