Section 68. We Now Come to Bone, a Tissue Confined to the

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cartilage (membrane bone), and the bone replacing the cartilage (cartilage bone) have, between them, practically superseded the cartilage altogether.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.