84 DEVELOPMENT OF VERTEBR.T]. 



"osseous," ossification is rarely so advanced as in the 

 higher vertebrata. In most of these 

 fishes, e. g.^ a deep cavity is left at each 

 end of the centrum (Fig. 8), cc^ which 

 cavity continues to be occupied by the 

 liquefied gelatinous remains of the 

 primitive notochord ; and the charac- 

 teristic of such element in a fish's 

 SECTION OF YERTEBR.K gj^eleton is, that it is " biconcave." Of 

 the minor amount of the earthy mat- 

 ter in the ossified parts of the skeleton of fishes, mention 

 has been already made ; and the consequent greater flexi- 

 bility and elasticity of such bones may be readily tested 

 by whoever will bend one of the long spines in the skele- 

 ton of a cod or turbot, and contrast its flexibility with 

 that of the similarly-shaped long and slender bone (puhisj 

 or fibula, e.g.) which he may find in the Christmas turkey 

 that follows in the feast. 



Two or more contiguous vertebrae are frequently sub- 

 jected to the same kind of modification, either by way of 

 excess or defect, and such groups of modified segments 

 have received special names; such, for example, as "skull" 

 {cranium), "neck" (cert'ia:), "chest" {thorax), "pelvis," and 

 "tail" (cft?/cfo); and these terms are reciprocally appli-ed, 

 when modified as adjectives, to the individual vertebrae 

 so grouped together, and which are called "cranial ver- 

 tebrae," "cervical vertebrae," "dorsal" or "thoracic ver- 

 tebra," " sacral" or " pelvic vertebrae," and " caudal ver- 

 tebrae." 



