112 



Basic Structure of Vertebrates 



BASIS PHENOID 

 ORBITOSPHENOID \ ALISPHENOID 

 PRESPHENOID \ \ \ PARIETAL 

 FRONTAL 

 VOMER 

 PREFRONTAL 

 NASAL 



SUPRAOCCIPITAL 



EXOCCIPITAL 

 , BASIOCCIPITAL 



MAMMAL 



Fig. 110. One of Owen's figures illustrating the Goethe-Oken vertebral theory 

 of the skull. Owen regarded the mammalian skull as constituted of four enlarged 

 and highly modified vertebrae (their centra shown in solid black). (After Wilder. 

 Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston 

 Company.) 



into the more complex brain. Similarly, the axial skeleton continues 

 forward and becomes elaborated into a suitable protective structure, 

 not only for the brain but also for the three pairs of important sense 

 organs of the head, the nasal organs, eyes, and ears. This anterior 

 terminus of the axial skeleton is the cranium, a part of the skeletal 

 complex known as the "skull." In the structure of the cranium are 

 features which suggested to the early anatomists that it is virtually a 

 series of expanded and modified vertebrae. Thus the atlas articulates 

 with the ventral portion of a complete ring constituted of four bones. 

 A massive midventral boue, the basioccipital, underlies the rear part 

 of the brain. At each side the basioccipital joins an exoccipital and the 

 ring is completed dorsally by the supraoccipital (Fig. 110). This 

 whole occipital arch of four bones can be imagined to be a vertebra, 

 its centrum being the basioccipital surmounted by a "neural arch'' 

 enclosing the central nervous organ. The arrangement of cranial bones 

 anterior to the occipital ring is such as to suggest a series of two or 

 three more modified vertebrae. The vertebral column is concerned pri- 

 marily with muscular action, but incidentally affords protection to the 

 overlying central nervous organ. The cranium serves mainly as protec- 

 tion for the brain and is to a relatively small extent concerned with 

 muscles. Leaving aside questions of interpretation, the cranium is me- 

 chanically or functionally a continuation of the vertebral column. The 

 main axial skeleton of a vertebrate consists of a longer vertebral part 

 which serves as the mechanical axis of neck, trunk, and tail, and a 

 shorter cephalic part, the cranium. 



