vertebrata, and typically shown only in the higher types. As we
descend in the scale from birds and mammals to lizards, amphibia
(frogs and toads) and fish, we find cartilage continually more
important, and the bony constituent of the skeleton correspondingly
less so. In such a type as the dog-fish, the skeleton is entirely
cartilaginous, bone only occurs in connection with the animal's
scales; it must have been in connection with scales that bone first
appeared in the vertebrate sub-kingdom. In the frog we have a
cartilaginous skeleton overlaid by numerous bony scutes (shield-like
plates) which, when the student comes to study that type, he will
perceive are equivalent to the bony parts of such scales as occur in
the dog-fish, sunk inward, and plating over the cartilage; and in the
frog the cartilage also is itself, in a few places, replaced by bony
tissue. In the adult rabbit these two kinds of bone, the bone overlying
what was originally cartilage (membrane bone), and the bone
replacing the cartilage (cartilage bone) have, between them,
practically superseded the cartilage altogether. The structure of the
most characteristic kind of bone will be understood by reference to
Figure XVI. It is a simplified diagram of the transverse section of
such a bone as the thigh bone. M.C. is the central marrow cavity,
H.v., H.v. are cross sections of small bloodvessels, the Haversian
vessels running more or less longitudinally through, the bone in
canals, the Haversian canals. Arranged round these vessels are
circles of the formative elements, the bone corpuscles or
osteoblasts (b.c.) each embedded in bony matrix in a little bed, the
lacuna, and communicating one with another by fine processes
through canaliculi in the matrix, which processes are only to be
seen clearly in decalcified bone (See Section 70). The osteoblasts are
arranged in concentric series, and the matrix is therefore in concentric
layers, or lamellae (c.l.). Without and within the zone of Haversian
systems are (o.l. and i.l.), the outer and inner lamellae. The bone is
surrounded by connective tissue, the periosteum. In addition to this
compact bone, there is a lighter and looser variety in which spicules
and bars of bony tissue are loosely interwoven. Many flat bones, the
bones of the skull, for instance, consist of this spongy bone, plated
(as an electro spoon is plated) with compact bone.