THE SKIN AND SKELETON. 145 



the roof of the nose. 8. Tlie two maxillce; that part of 

 the upper jaw in which the canines, premolars, and molars 

 are lodged. 9. The two premaxiUce, in which the upper 

 incisors are situated. 10. The two ^atomes, which, with 

 the maxillary Iwnes, form the roof of the month. 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 

 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 vertebrae, or bones of the neck, are peculiar 

 iu 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 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 lurrihar vertebrae are the massive vertebrae lying in 

 the loins between the dorsals and the hip-bones. 



The sacral vertebrae lie between the hip-bones, and are 



10 



