90 VERTEBRATED ANIMALS. 



are the six parts which comparative anatomists consider 

 the elements of the vertebrae, and which are found in 

 most vertebrated animals, however various in form, size, 

 or habits, they may be. In some cases however, in ad- 

 dition to these, there is the process f, bifid at the base, 

 and forming a spine at the lower surface of the verte- 

 bra, or opposite to the spinous process. This structure 

 is common in the Fishes. The aperture formed by the 

 bifurcation of this process admits the passage of a large 

 artery. 



As our plan will not allow an extension of this part 

 of our subject to the different orders of the vertebrata, 

 we will omit any illustration from the quadrupeds, as 

 being most nearly allied to man in the scale of organi- 

 zation, and therefore most likely to present similar 

 spines; and since illustrations the most remote from 

 man in the scale of being, will tend most clearly to show 

 a unity of plan in the construction of the whole, we 

 will give examples from some of the lower orders of 

 vertebrated animals. 



Vertebra of Fish. Fig 67 represents a section of a 

 Fig. 67. part of the spine of a Fish 



standing in the natural posi- 

 tion. The body of each ver- 

 tebra is hollowed out on both 

 sides, so as to form cup like 

 cavities ; by which means, 

 when the two convex cavities 

 are applied to each other as in 

 the living animal, a cavity hav- 

 ing the shape of a short double 

 cone is formed, as shown in 

 the figure. These cavities left by the bones, are filled 

 with a gelatinous substance, which is nearly incompress- 

 ible, and which appears to serve as a kind of pivot for 

 the motions of the joint. By dividing a spine in the 

 centre, these parts are seen, as shown in the figure. 

 A single vertebra is represented by Fig. 68, for the 



Explain Fig. 67. What is the difference between the vertebrae of 

 quadrupeds and fishes'? 



