STKUCTUEE OF THE SKULL. 53 



The surface of each bone is covered with a membrane 

 called the periosteum, as already stated. 



The surfaces of bones present elevations or processes, and 

 depressions, some of which are for the attachment of muscles. 



Some of the bones, as those of the face, are fixed firmly 

 together by mere closeness of position ; others are united, 

 as in the head, by sutures ; others are implanted, as the 

 teeth, if we regard teeth as bones (see page 5G) ; and 

 others, as the main bones of the trunk and limbs, are held 

 together by ligaments. 



The Skeleton is divided into three well-defined regions, 

 as the head or skull, trunk, and extremities. 



The skull consists mainly of the cranium or brain-case, 

 together with parts protecting and supporting the organs 

 of hearing, sight, and smell, the whole formed of numer- 

 ous bones, closely united by sutures, or by anchylosis (Figs. 

 61, 62, 63). To the cranium, on the lower surface, are at- 

 tached the lower jaw or ma/ndible as it is called by anato- 

 mists and the hyoidean apparatus (sh* eh* c/i, bh, A,Fig. 62). 



The form and structure of the skull of one of the most 

 common of the Mammals, namely, the Dog, are shown in 

 the three following wood-engravings (Figs. 61, 62, 63), 

 from Flower's " Osteology of the Mammalia," an excel- 

 lent hand-book for the pupil who would gain more infor- 

 mation of the Mammalian skeleton than our space here 

 allows us to give. The names, however, of the different 

 parts of the skull, are annexed to the engravings, as they 

 stand in Flower's work just named. 



The skull in all the Mammalia has two occipital condyles 

 (oc, Fig. 63) or articulating surfaces, by which it is articu- 

 lated to the first vertebra or atlas of the neck, instead of 

 one articulating surface or condyle, as in Birds and Reptiles. 



In all the Mammals, the upper jaw is fixed to the 



