48 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



These are separate ossifications which only unite with the main bone at the time 

 the adult condition is reached. The increase in diameter has some interesting 

 features. In animals fed with madder, the bone formed during the feeding is 

 colored. In this way it is found that the new bone (fig. 40, A) is laid down on the 

 outside of the other, and that with growth (J5 and C) ; the 'marrow cavity' on 

 the inside is increased in size by the resorption of the bone already formed. 



For convenience of treatment the endoskeleton is divided into 

 axial and appendicular portions, the axial consisting of the vertebral 

 column (backbone) and the skull, together with the ribs and sternum 

 which are closely associated with the vertebrae. The appendicular 

 skeleton includes the framework of the limbs and fins and the 

 girdles to which they may be attached. 



Axial Skeleton 



Both the skull and the vertebral column surround and protect 

 the central nervous system (brain and spinal cord) and in this way 

 the skull is an enlarged and specialized portion of a continuous axis, 

 but it is not possible to carry the comparisons into details. The idea 

 of Oken and Owen that the skull is a complex of three or four vertebrae 

 has long been overthrown. The skull dififers markedly from the 

 vertebral column in the presence of numerous membrane bones, 

 while in the more primitive vertebrates and in the early stages of 

 all, the Okenian vertebrae are entirely lacking. 



Vertebral Column 



The vertebral column ('backbone') is the longitudinal skeletal 

 axis of the body behind the head. It lies dorsal to the alimentary 

 canal and most of it is ventral to the spinal cord, only the neural arches 

 arising above the central nervous system. In all vertebrates, except 

 some of the cyclostomes, it is markedly metameric, being composed 

 of a series of elements — the vertebrae — which are always preformed 

 in cartilage, but which, in the higher groups, are ossified later. 



The most typical vertebrae are found in the tails of the lower 

 vertebrates (fig. 41, A). Each vertebra consists of several por- 

 tions or regions. Between the spinal cord and the caudal blood- 

 vessels, and surrounding the notochord is the body or centrum, the 

 successive vertebrae of the column articulating by the anterior and 

 posterior faces of the centra. From the dorsal side of each centrum 

 a neural arch arises, enclosing the spinal cord in its opening, the 



