is intercellular in character, and corresponds to the matrix of cartilage.
Bone may be compact, or loose and spongy in character, when it is known as cancellous bone. In compact bone many of the lamellae are arranged concentrically round cavities, the Haversian canals, which in life are occupied by blood-vessels. Each Haversian canal with its lamellae forms a Haversian system. In spongy bone instead of Haversian canals there occur large irregular spaces filled with marrow, which consists chiefly of blood-vessels and fatty tissue. The centre of a long bone is generally occupied by one large continuous marrow cavity. The whole bone is surrounded by a fibrous connective tissue membrane, the periosteum.
The development of bone.
Periosteal ossification. An example of a bone entirely formed in this way is afforded by the parietal. The first trace of ossification is shown by the appearance, below the membrane which occupies the place of the bone in the early embryo, of calcareous spicules of bony matter, which are laid down round themselves by certain large cells, the osteoblasts. These osteoblasts gradually get surrounded by the matter which they secrete and become converted into bone cells, and in this way a mass of spongy bone is gradually produced. Meanwhile a definite periosteum has been formed round the developing bone, and on its inner side fresh osteoblasts are produced, and these with the others gradually render the bone larger and more and more compact. Finally, the middle layer of the bone becomes again hollowed out and rendered spongy by the absorption of part of the bony matter.
Endochondral ossification[5]. This is best studied in the case of a long bone like the femur or humerus. Such a long bone consists of a shaft, which forms the main part, and two terminal portions, which form the epiphyses, or portions ossifying from centres distinct from that forming the shaft or main part of the bone.
In the earliest stage the future bone consists of hyaline cartilage surrounded by a vascular sheath, the perichondrium.
Then, starting from the centre, the cartilage becomes permeated by a number of channels into which pass vessels from the perichondrium and osteoblasts. In this way the centre of the developing shaft becomes converted into a mass of cavities separated by bands or trabeculae of cartilage. This cartilage next becomes calcified, but as yet is not converted into true bone. The osteoblasts in connection with the cavities now begin to deposit true endochondral spongy bone, and then after a time this becomes absorbed by certain large cells, the osteoclasts, and resolved into marrow or vascular tissue loaded with fat. So that the centre of the shaft passes from the condition of hyaline cartilage to that of calcified cartilage, thence to the condition of spongy bone, and finally to that of marrow. At the same time beneath the perichondrium osteoblasts are developed which also begin to give rise to spongy bone. The perichondrium thus becomes the periosteum, and the bone produced by it, is periosteal or membrane bone. So that while a continuous marrow cavity is gradually being formed in the centre of the shaft, the layer of periosteal bone round the margin is gradually thickening, and becoming more and more compact by the narrowing down of its cavities to the size of Haversian canals. The absorption of endochondral and formation of periosteal bone goes on, till in time it comes about that the whole of the shaft, except its terminations, is of periosteal origin. At the extremities of the shaft, however, and at the epiphyses, each of which is for a long time separated from the shaft by a pad of cartilage, the ossification is mainly endochondral, the periosteal bone being represented only by a thin layer.
Until the adult condition is reached and growth ceases, the pad of cartilage between the epiphysis and the shaft continues to grow, its outer (epiphysial) half growing by the formation of fresh cartilage as fast as its inner half is encroached on by the growth of bone from the shaft. The terminal or articular surfaces of the bone remain throughout life covered by layers of articular cartilage.
Even after the adult condition is reached the bone is subject to continual change, processes of absorption and fresh formation going on for a time and tending to render the bone more compact.
Methods in which bones are united to one another.
The various bones composing the endoskeleton are united to one another either by sutures or by movable joints.
When two bones are suturally united, their edges fit closely together and often interlock, being also bound together by the periosteum.
In many cases this sutural union passes into fusion or ankylosis, ossification extending completely from one bone to the other with the obliteration of the intervening suture. This feature is especially well marked in the cranium of most birds.
The various kinds of joints or articulations[6] may be subdivided into imperfect joints and perfect joints.
In imperfect joints, such as the intervertebral joints of mammals, the two contiguous surfaces are united by a mass of fibrous tissue which allows only a limited amount of motion.
In perfect joints the contiguous articular surfaces are covered with cartilage, and between them lies a synovial membrane which secretes a viscid lubricating fluid.
The amount of motion possible varies according to the nature of the articular surfaces; these include—
a. ball and socket joints, like the hip and shoulder, in which the end of one bone works in a cup provided by another, and movements can take place in a variety of planes.
b. hinge joints, like the elbow and knee, in which as in ball-and-socket joints one bone works in a cup provided by another, but movements can take place in one plane only.
THE ENDOSKELETON.
The endoskeleton is divisible into axial and appendicular parts; and the axial skeleton into—
1. the spinal column,
2. the skull {a. the cranium, {b. the jaws and visceral skeleton, 3. the ribs and sternum[7].
I. The Axial Skeleton.
1. The Spinal column.
The spinal column in the simplest cases consists of an unsegmented rod, the notochord, surrounded by the skeletogenous layer, a sheath of mesoblastic origin, which also envelops the nerve cord. Several intermediate stages connect this simple spinal column with the vertebral column characteristic of higher vertebrates. A typical vertebral column may be said to consist of (1) a series of cartilaginous or bony blocks, the vertebral centra, which arise in the sheath surrounding the notochord. They cause the notochord to become constricted and to atrophy to a varying extent, though a remnant of it persists, either permanently or for a long period, within each centrum or between successive centra. (2) From the dorsal surface of each centrum arise a pair of processes which grow round the spinal cord and unite above it, forming a dorsal or neural arch. (3) A similar pair of processes arising from the ventral surface of the centrum form the ventral or haemal arch. To the ventral arch the ribs strictly belong, and it tends to surround the ventral blood-vessels and the body cavity with the alimentary canal and other viscera.
A neural spine or spinous process commonly projects upwards from the dorsal surface of the neural arch, and a pair of transverse processes project outwards from its sides. When, as is commonly the case, the two halves of the haemal arch do not meet, the ventral surface of the centrum often bears a downwardly-projecting hypapophysis.
The character of the surfaces by which vertebral centra articulate with one another varies much. Sometimes both surfaces are concave, and the vertebra is then said to be amphicoelous; sometimes a centrum is convex in front and concave behind, the vertebra is then opisthocoelous, sometimes concave in front and convex behind, when the vertebra is procoelous. Again, in many vertebrae both faces of the centra are flat, while in others they are saddle-shaped, as in the neck vertebrae of living birds, or biconvex, as in the case of the first caudal vertebra of crocodiles.