106 Basic Structure of Vertebrates 



This embryonic sequence of skeletal materials and the very indi- 

 rect mode of development of a backbone raise perplexing questions. 

 Why should a notochord develop in mammals, even in man? Why 

 should a somewhat elaborate cartilaginous vertebral column be built 

 up in the embryo only to be obliterated and replaced by bone? Bones 

 can develop without being preformed in cartilage. 



VERTEBRAL COLUMN 



A single unit of the backbone is a vertebra. The structure of verte- 

 brae varies depending on the region of the vertebral column in which 

 the vertebra occurs and the animal to which the column belongs. Cer- 

 tain basic features, however, are common to all vertebrae except those 

 which are more or less reduced (e.g., the vestigial vertebrae in the dis- 

 tal region of a tail), or those which become specialized in "relation to 

 some local peculiarity of function in the column (e.g., the one or two 

 anterior vertebrae of the neck, which are concerned with the support 

 and movement of the skull). Cartilaginous vertebrae, such as those of 

 sharks, have the same general form as bony vertebrae. 



The main body of a vertebra is the centrum, usually solid and ap- 

 proximately cylindric (Figs. 102, 103, 104). It lies ventral to the spinal 

 cord and in the position of the embryonic notochord, which it replaces. 

 The articulating surfaces of centra may be concave, convex, or flat. 

 An amphicoelous centrum has both surfaces concave (as in most 

 fishes) ; a procoelous centrum is concave in front; an opisthocoelous 

 centrum is concave behind; if flat at both ends, it is called "am- 

 phiplatyan" (Fig. 104). The spaces between successive amphicoelous 

 centra may be occupied by persisting remnants of the embryonic noto- 

 chord. Dorsally, a centrum is surmounted by a neural arch through 

 whose aperture, the vertebral canal, extends the spinal cord. The 

 apex of the arch is more or less prolonged upward to form a neural 

 spine. Caudal vertebrae also usually have a henial arch depending 

 from the ventral surface of the centrum and enclosing the caudal ar- 

 tery and vein (Fig. 103). The hemal arch may be prolonged downward 

 to form a hemal spine. 



In a cartilaginous vertebral column such as occurs in sharklike 

 fishes, the spaces between successive neural arches are filled by car- 

 tilaginous plates known as interdorsals (Figs. 105, 106). Similarly, in 

 the tail, the spaces between hemal arches are filled by interventral 

 cartilages. All of the parts in these cartilaginous columns are closely 

 tied together by fibrous connective tissue. Consequently, the spinal 

 cord and caudal blood-vessels are completely surrounded by vertebral 

 structures. There are, in a strict sense, no joints in such a vertebral 

 column. Bending is not localized at intervertebral joints but involves 



