124 Basic Structure of Vertebrates 



of vertebrates. Such skulls as those of a codfish, frog, snake, bird, and 

 whale are so different in appearance that a casual observer could 

 hardly suspect that they possess any basic similarity. The truth is, 

 however, that careful analysis of skulls, different as they may be, 

 reveals a general plan or "pattern" which is common to all of them. 

 Bones which may be identified as corresponding parts of various 

 skulls maintain constant spatial relations to one another. The differ- 

 ences between two skulls are due mainly to differences in shape and 

 size of corresponding bones or to addition or omission of bones, but 

 not to changes in the spatial relations of the bones that are present. 

 The "pattern" is constant, but its constituent parts are highly modi- 

 fiable. The present purpose will be best served by describing the 

 "pattern" rather than any particular skulls. 



Throughout the following description of the bones of the skull, 

 constant reference should be made to the diagrams (Figs. 120, 121), 

 and to Table 1 related to them (see p. 122). 



Bones of Cranium 



In the bony cranium, as already mentioned, may be recognized 

 a series of segments or, better, "arches," since they certainly do not 

 correspond to the segments of the vertebral column or body-muscle 

 (Fig. 110). That they do not is proved by the fact that, during em- 

 bryonic development of the chondrocranium, several of the adjacent 

 cartilaginous vertebrae ("occipital vertebrae") fuse with the hind 

 end of the cranium and with one another, eventually being completely 

 and unrecognizably incorporated into its occipital region (Fig. 116). 



At the posterior end of the cranium is the occipital arch, consist- 

 ing of a basioccipital, a pair of exoccipitals, and a supraoccipital. 

 These are all cartilage bones except that the supraoccipital may be 

 augmented by dermal ossification. Anterior to the basioccipital is a 

 basisphenoid flanked laterally by a pair of alisphenoids. These 

 three are cartilage bones. The arch is completed dorsally by a pair of 

 dermal bones, the parietals — hence it is called the "parietal arch." 

 Anterior to the basisphenoid may be a narrow median presphenoid 

 joined laterally to a pair of orbitosphenoids, which form a part of 

 the wall of the orbit. The three sphenoids are cartilage bones and this 

 frontal arch is completed by a dorsal pair of dermal bones, the 

 frontals. The frontals lie over the region of the orbit. The remaining 

 anterior or nasal region of the cranium (Fig. 120, 19, 20) is not so 

 definitely archlike. The embryonic cartilage ossifies to form a some- 

 what complex group of ethmoid bones, the mesethmoid being the 

 partition between right and left nasal cavities and an ectethmoid 

 forming the upper part of the wall of each cavity. The nasal region is 



