fact become bone corpuscles. Increase in thickness of the new bone is effected by the deposition of fresh matrix followed again by the inclusion of further osteoblasts. The spaces within the trabeculae become in this way gradually narrowed by the deposition of matrix until at last only a narrow centre is left large enough to contain the blood vessels and their accompanying nerves, lymphatics and a small number of osteoblasts. Bone formation then ceases. In this manner the Haversian systems are produced.
Growth of the bone proceeds by the deposition of more matrix on the exterior, but simultaneously a process of absorption is also taking place.
This is most typically seen within the spongy portion of the bone. The absorption of the trabeculae is effected by the osteoclasts. These become applied to the trabeculae and gradually eat their way into the matrix thus coming to lie within lacunae. They possess the power of dissolving both bone and cartilage matrix. Side by side with this solution process we may often see new formation taking place by the activity of the osteoblasts (fig. 10). In this manner the whole framework of the bone may be gradually replaced. The process is most active in embryos and very young animals, but also continues during the whole life of an animal, thus effecting alterations in shape and structure of the whole bone. Growth in the length of a bone is effected by formation of new bone at either end of the shaft. After the ossification centre has been formed in the shaft (diaphysis) of the bone subsidiary centres make their appearance in the heads of the bones. These form, by a similar process of bone formation, fresh bone masses which, however, are not continuous with the bone tissue of the shaft. They form the epiphyses. They are attached to the diaphysis by an intermediate piece of cartilage, and it is by a process of growth of this cartilage and its subsequent replacement by bone that growth in length of the whole bone is effected (fig. 10). This piece of intervening cartilage can be easily seen in a young bone and persists as long as the bone can increase in length. Thus in man the last junction of epiphysis to diaphysis may not take place until the 28th year.
Development of bone in membrane shows a course in all respects very similar to perichondral bone formation. A layer of osteogenetic tissue makes its appearance in the membrane from which the bone is to be formed. In this tissue a number of stiff fibres are deposited which soon become covered and impregnated with calcium salts. Around these bundles of fibres numbers of osteoblasts are deposited and by them bone matrix is deposited in irregular trabeculae. The bone increases by the deposition of fresh matrix just as in perichondral bone formation and Haversian systems are formed after precisely the same manner as in that position. The factor determining the position of one of these systems is of course the presence of a blood vessel penetrating towards the deeper part of the bone.
Muscle.—Muscle is the contractile tissue of the body, that tissue by which the various parts of the body are moved. Thus it forms the main bulk of the limbs, back, neck and body wall. Most of the viscera too possess well-developed muscular coats. When separated into its constituent parts it is seen that muscle in all instances is built up of a number of long fibres. These are of three well-defined types. Those forming the skeletal muscles are of large size, even in some instances up to 12 cms. in length, their diameter varying from 0,01 to 0,1 mm. When these are examined under the microscope they are found to be characterized by possessing a decided transverse marking, and they are therefore known as striated muscle fibres. From the fact that they comprise those muscles which are under the control of the will they are also called voluntary muscle fibres. The second variety of muscle is made up of much smaller fibres varying in different parts from 0,05 to 0,15 mm. in length and about 0,005 mm. in diameter. These fibres show no transverse striations nor are they directly under the control of the will. They are therefore termed smooth or involuntary muscle. Lastly, there is a third type of muscle found in the heart which lies intermediate in structure between these two varieties. In this the fibres are small and show distinct transverse striations. Longitudinal striations are also present though somewhat less marked. In most respects this form of muscle fibre resembles smooth muscle more closely than striated muscle.
Voluntary or Striated Muscle.—Each muscle fibre of which this is composed is what is known as a syncytium or plasmodium, i.e. a structure containing a number of nuclei, which has been formed from a single cell by proliferation of its nucleus without subdivision of the protoplasm. It is thus an assemblage of cells possessing a common protoplasm. Each fibre generally runs parallel to the length of the muscle and if that muscle is short extends the whole length. Thus the one end of the fibre may be attached to tendon when the end is rounded off. The other end may also terminate in tendon or in the fibrous covering of bone in which case it is again rounded. In long muscles, however, the fibre may only extend a certain distance along the muscle, and it is then found to terminate in a tapering or bevelled end. In some of the long muscles some of the fibres may both arise and terminate in the substance of the muscles. In such a case both ends are bevelled. All the fibres in a muscle are arranged parallel to one another.
The outer surface of each muscle fibre consists of a tough homogeneous membrane, the sarcolemma. The main muscle substance (see fig. 11) is composed of several parts, viz. the fibrillae, the sarcoplasm and the nuclei. Under the action of reagents the muscle fibre may be split into a number of longitudinal elements. These are the fibrillae. They possess alternate bands of light and dark substance which give them a striated appearance. When viewed under polarized light the dark substance is found to be doubly refracting or anisotropic, the light band is singly refracting or isotropic.
|Fig. 11.—Striated or Voluntary muscle Fibre, with alternate light and dark bands and many nuclei immediately beneath the sarcolemma.|
According to many observers, in the centre of each isotropic segment there is a thin transverse disk of anisotropic material and in the centre of each anisotropic segment a thin disk of isotropic substance. The fibrillae are arranged in the muscle fibre parallel to one another and with the alternate light and dark bands at approximately the same level across the fibre, thus giving to the whole muscle fibre its typical transverse striation. The fibrillae are united to one another by interfibrillar substance to form bundles, of which there may be a considerable number in each muscle fibre. The bundles lie in a surrounding layer of sarcoplasm which apparently represents the remaining portion of unaltered protoplasm of the syncytium. This structure of muscle is best seen in the transverse sections of the fibres. A number of areas separated by a clear protoplasm are then to be seen. The areas are formed by the bundles of fibrillae seen in transverse section,