136 



Basic Structure of Vertebrates 



Fig. 129. Tails of fishes. (A) Young Amia. (h) Hypurals; (n) notochord; (s) spinal 

 cord. (B) Diphycercal. (C) Heterocercal. (D) Homocercal. (Courtesy, Kingsley: 

 "Comparative Anatomy of Vertebrates," Philadelphia, The Blakiston Company.) 



Skeletal Axes 



In descriptions of the vertebrate skeleton, it is the common prac- 

 tice to define the "axial skeleton" as consisting of the vertebral col- 

 umn, skull, ribs, and sternum — that is, the entire skeleton is "axial " 

 except such parts as constitute the "appendicular skeleton." Some 

 authors make three divisions of it — axial, visceral, and appendicular. 

 In either case, "axial" obviously refers to the vertebral or notochordal 

 axis. The division of the skeleton into two parts, axial and appendicu- 

 lar, has the virtue of simplicity, but otherwise it seems inadequate 

 and lacking in significance. 



The vertebrate body has more than one axis. Built on the "tube- 

 within-a-tube " plan (see p. 23 and Fig. 16), the inner or alimentary 

 tube establishes a visceral axis. The elongated coelom has a coelomic 

 axis. If the digestive tube, straightened out, is regarded as being the 

 central organ in the coelom, the visceral and coelomic axes coincide. 

 The basic structural — i.e., "architectural" — axis of the animal-as-a- 

 whole is the axis of the inner tube, the visceral axis. (Nutrition is 

 certainly the basic physiologic axis of the animal!) But, regarding the 

 body as a dynamic thing, its chief mechanical axis is occupied by the 

 vertebral column which is situated in the dorsal body-wall, whose 

 heavy musculature effects the major movements of the body. 



The visceral arches develop around the alimentary tube. They 

 are "axial," but it is in relation to the visceral axis. It is true that the 

 dorsal part of the first visceral arch becomes secondarily joined to the 

 floor of the cranium. But, in doing so, the bones of the upper jaw do 

 not cease to be visceral. The floor of the cranium is also the roof of 



