OR OSSEOUS SYSTEM. 59 



form of a pair of ribs to encompass the organs of this 

 part of the trunk. The vertebral elements situate above 

 the body of the bone expand in the region of the head 

 in the same manner as we here see those below the 

 cyclo-vertebral element in the region of the abdomen ; 

 and this they do in order to encompass the soft parts 

 contained in the cavity of the skull, and in the face. 

 Another position of these vertebral pieces, which is 

 common in the caudal region of the column in higher 

 classes, especially among the long-tailed reptiles, and in 

 cetaceous mammalia, is represented in the diagram C of 

 Fig. 28, where we observe the large foramen for the 

 nervous columns (/,) above the cyclo-vertebral element (a,) 

 requiring the whole extent of the two peri-vertebrals (b, b,) 

 for its formation, and the strong superior spinous pro- 

 cess is composed of the two epi-vertebrals placed side 

 by side. In the lower part of the same vertebra the 

 inferior laminae extend outwards to form strong trans- 

 verse processes (/,/,) and an inferior spinous process, 

 and an inferior foramen for the aorta (i,) and the vena 

 cava (#,) are formed by the approximation of the two 

 cata-vertebrals ( g, gj) which show their displacement by 

 being generally thrust backwards between two cyclo-ver- 

 tebral elements. When the two cata- vertebral elements are 

 extended outwards to form ribs in fishes, we very often 

 find them bifurcated, as represented in Fig. 28. B, g, g< 

 The general form of the vertebral elements is very much 

 modified and varied in different parts of the column by 

 the shape and magnitude of the parts these elements 

 embrace, and the extent of surface for muscular attach- 

 ments which they require to present. The epi- and peri- 

 vertebrals are most expanded in the skull and the sacrum, 

 and the para- and cata-vertebrals, where they embrace 

 the viscera of the trunk. The appearance of the entire 

 skeleton of vertebrated animals is greatly varied by the 

 difference in the position of the ribs, or of that part of 

 the column where the para- and cata-vertebral elements 

 are extended over the great viscera of the trunk. In 

 fishes, as shown in the annexed figure, (Fig. 29. 3,) and 

 in cetaceous mammalia, the fixed, ribbed, and thoracic 

 part of the column (,) is placed near its anterior extre- 



