THE FROG AND VERTEBRATES IN GENERAL 175 



spaces." The osteoblasts lying next to the periosteum continue 

 to add new bone during development so that the bone increases 

 in diameter by the addition of outside layers. The bone tissue 

 may also increase to a certain extent by the addition of bony tissue 

 in the marrow cavity. (W. f. 32, D; 100.) 



4. Skeleton 



The living bony endoskeleton of a Vertebrate may be divided 

 into (i) the axial skeleton consisting of the skull and verte- 

 bral column, and (u) the appendicular skeleton consisting 

 of the limbs and the limb girdles. (W. pp. 138-144.) 



I. AXIAL SKELETON 



A. The Skull. The Vertebrate skull consists of two portions : 

 the cranium, which is the brain case, and the underlying facial 

 portion. Making up the skull is a considerable number of bones as 

 well as certain permanently cartilaginous elements. The cranium 

 in the Frog is small, narrow, and elongated in the direction of the 

 long axis of the body. It encloses and protects the brain, together 

 with the auditory and olfactory sense organs. At the posterior end 

 of the cranium there is a large opening, the foramen magnum, 

 through which the spinal cord leaves the cranium and passes into 

 the vertebral column. Surrounding the foramen magnum are the 

 exoccipital bones, each of which bears a rounded prominence, or 

 occipital condyle, which articulates with the first vertebra and 

 thus forms the connection between the skull and the vertebral 

 column. Just anterior and lateral to each exoccipital bone is a 

 prootic bone. Lying in the anterior portion of the cranium and 

 also forming the posterior wall of the nasal cavity is an unpaired 

 bone known as the sphenethmoid. The five bones just noted ; 

 that is, a pair of occipitals, a pair of prootics, and the unpaired 

 sphenethmoid, constitute the cartilage bones of the Frog's cranium. 

 (W. f. 103.) 



There are also a number of membrane bones which form the 

 dorsal covering of the cranium. There is a pair of long, narrow 

 bones, the front al-parietals, which result from a fusion of two 

 pairs of bones, as commonly found in Vertebrates, namely, the 

 front als and parietals. The frontal-parietal bones connect 

 posteriorly with the prootics and the exoccipitals, anteriorly with 

 the sphenethmoid, and form the greater portion of the dorsal wall 



