OR OSSEOUS SYSTEM. 6'1 



are filled up each with a short sac of a gelatinous thin 

 fluid, as seen in Fig. 30. C. c, so that these vertebrae 

 play freely over the surface of so many 

 elastic interposed balls. The bodies of the 

 vertebrae are the elements first developed 

 in the animal kingdom. They are the 

 most important, as means of support ; 

 they are the parts most developed in the 

 vertebrae of fishes, and the skeletons of 

 the lowest cartilaginous species, are com- 

 posed almost solely of these elements. 

 Even in the sharks, the other elements 

 are remarkably small and are not ajichy- 

 losed to the bodies of the vertebrae ; so that by macera- 

 tion they fall off, and leave deep depressions ( Fig. 30. 

 B, c.) in the sides of the cyclo-vertebral elements where 

 they were attached. By the great size of the central 

 passage in the bodies of the vertebrae of many cartila- 

 ginous fishes, the inter-vertebral substances communicate 

 with each other, and form a continuous elastic beaded 

 chord passing through the whole vertebral column, as in 

 the lampreys. The component elements of the vertebrae 

 of fishes are disposed in such a manner as to give great 

 vertical extension to the trunk, and thus to form a broad 

 lateral surface to strike the water in their horizontal mode 

 of progression. As the spinal chord is small, the upper 

 vertebral foramen is also small in fishes, and the two 

 superior laminae therefore soon meet to form a spinous 

 process by their junction, as seen in the skeleton of the 

 perch, (Fig. 31. c,c, c.) Along the greater portion of the 

 back, we observe the two epi-vertebral elements (Fig. 3 1 . 

 74, 75,) placed end to end in a vertical direction, the 

 short inferior portion forming the interspinous bone. (74,) 

 and the more slender superior portion extending from the 

 trunk and covered with a prolonged fold of the skin, 

 forms a ray (75,) of the dorsal fin, and thus are con- 

 structed all the dorsal fins placed along the middle of 

 the back from the head to the tail. The same two ele- 

 ments in several of the last cocygeal vertebrae, pass back- 

 wards in a very oblique manner, and constitute the cau- 

 dal fin (70, 78.) The corresponding elements below the 



