THE SKIN AND SKELETON. 145 



the nose. 8. The maxilla; that part of the upper jaw in 

 which the canines, premolars, and molars are lodged. 9. 

 The premaxilla, in which the upper incisors are situated. 

 10. The palatine, which, with the maxillary bones, forms 

 the roof of the mouth. There are two appendages to the 

 skull: the mandible, or lower jaw, whose condyles, or' 

 rounded extremities, fit into a cavity (the glenoid) in the 

 temporal bone ; and the hyoid, 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 vertebrae, 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 in the last one. The 

 number of cervical vertebras ranges from 1 in the Frog 

 to 25 in the Swan. 



The dorsal vertebrae 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 vertebrce 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 

 10 



