10 
FISHES. 
yet in some of the Spiny-finned Fishes they 
possess much hardness. In one large division, 
including the Sharks and Rays, the skeleton is 
composed of gristle or cartilage instead of bone. 
The vertehrcB, or joints of the spine, are excavated 
at each end in a conical cavity ; the hollow thus 
formed between every joint and its neighbour is 
filled with a jelly-like substance, which is con- 
tinuous through the whole spine, by means of a 
hole pierced through the centre of each vertebra. 
There is no true spinal marrow. In general, the 
tubular perforation is small, but in many of the 
Gristly Fishes it is of so great a diameter as to 
reduce the vertebrce to mere cartilaginous rings. 
The vertehrce give origin to spinous processes, 
both above and below, for the attachment of 
muscles. Within the cavity of the belly the 
lower processes are wanting, and are replaced 
by lateral ones, to which the ribs are attached. 
These are commonly numerous, slender, flexible 
bones, each of which sends off a branch of almost 
equal length and tenuity ; some species, as the 
