Aim and Method of Comparative Anatomy 



377 



ever, reveals close similarity. In all bony skulls, the bones which con- 

 stitute the greater part of the cranium are disposed in the form of three 

 arches, each of which completely encircles the brain. Named in order, 

 beginning at the rear of the cranium, these arches are the occipital, 

 parietal, and frontal (Fig. 298). Each arch consists of four or five bones. 

 A mid-ventral bone is joined to one lateral bone on each side and the 

 arch is completed dorsally by one or a pair of bones (the supraoccipital, 

 the two parietals, the two frontals). The early anatomists recognized 



BASIS PKENOID 

 ORBITOSPHENOID \ ALISPHENOID 

 PRESPHENOID \ \ \ PARIETAL 

 FRONTAL 

 VOMER 

 PREFRONTAL. 

 NASAL 



SUPRAOCCIPITAL 



[OCCIPITAL 

 BASIOCCIPITAL 



Fig. 298. One of Owen's figures illustrating the 

 Goethe-Oken vertebral theory of the skull. Owen inter- 

 preted the mammalian skull as constituted of four en- 

 larged and modified vertebrae. Not knowing the embryol- 

 ogy of the skull, he did not realize that vertebrae lack 

 the membranous bones which constitute so large a part 

 of the skull. (After Wilder. Courtesy, Neal and Hand: 

 "Chordate Anatomy," Philadelphia, The Blakiston 

 Company.) 



these arches and, led by Goethe, interpreted them as modified verte- 

 brae. Enthusiasm for complete "'unity of plan" led to more or less 

 dubious recognition of additional " arches," but with wide disagreement 

 as to their number and constitution. The "vertebral theory" was long 

 since abandoned, but the three arches mentioned above are neither 

 imaginary nor theoretic. As for the remaining bones of the skull, if not 

 arranged in the form of arches they are at least arranged in very defi- 

 nite and constant relations to the inner ear, eye, and nasal cavities and 

 to the jaws and branchial structures (Fig. 120). 



The very striking differences in the form of skulls depend not on 

 change of plan of structure but on alteration in the form of the indi- 

 vidual bones. Imagine a skull of average proportions modeled in soft 

 rubber. Let the boundaries (sutures) of bones be represented by lines 

 painted on the rubber. By application of appropriate tensions and 

 pressures, either to the model as a whole or locally, it could be made to 



