Page:Natural History, Fishes.djvu/24

10 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 vertebræ, 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 continuous 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 vertebræ to mere cartilaginous rings.

The vertebræ 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