354 COMPARATIVE ZOOLOGY 



process to meet one from the squamosal, forming the 

 zygomatic arch. 7. The two nasals, forming the roof 

 of the nose. 8. The two maxillce, that part of the 

 upper jaw in which the canines, premolars, and molars 

 are lodged. 9. The two premaxillce, in which the 

 upper incisors are situated. 10. The two palatines, 

 which, with the maxillary bones, form 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 tem- 

 poral 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, inclosing 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 cartilage remains inside the bones. In higher ver- 

 tebrates the cartilage never makes an entire box, and 

 early disappears. 



The cervical vertebra, or bones of the neck, are pecul- 

 iar 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 

 second, called axis, around a blunt peglike projection, 

 called the odontoid process. The centra are usually 

 wider than deep, and the neural spines very short, ex- 

 cept on the last one. The number of cervical vertebrae 

 ranges from I in the frog to 25 in the swan. 



The dorsal vertebra are such as bear ribs, which, unit- 

 ing with the breastbone, or sternum, form a bony arch 

 over the heart and lungs, called the thorax. The ster- 

 num may be wanting, as in fishes and snakes, or greatly 

 developed, as in birds. When present, the first verte- 

 bra whose ribs are connected with it is the first dorsal. 



