_osseous_ and the _cartilaginous_, yet the distinction is not very
precise; for the first have a great deal of cartilage, and the second,
at any rate, a portion of calcareous matter in their bones. It may,
therefore, be said that the bones of fishes form a kind of intermediate
substance between true bones and cartilages. The backbone extends
through the whole length of the body, and consists of vertebrae, strong
and thick towards the head, but weaker and more slender as it approaches
the tail. Each species has a determinate number of vertebrae, which are
increased in size in proportion with the body. The ribs are attached to
the processes of the vertebrae, and inclose the breast and abdomen. Some
kinds, as the rays, have no ribs; whilst others, as the sturgeon and
eel, have very short ones. Between the pointed processes of the
vertebrae are situated the bones which support the dorsal (back) and the
anal (below the tail) fins, which are connected with the processes by a
ligament. At the breast are the sternum or breastbone, clavicles or
collar-bones, and the scapulae or shoulder-blades, on which the
pectoral or breast fins are placed. The bones which support the ventral
or belly fins are called the _ossa pelvis_. Besides these principal
bones, there are often other smaller ones, placed between the muscles to
assist their motion.