muscles. Most of the skeleton of the fetus consists of cartilage, which later develops into bone.
Bone.—In bone the intercellular tissue is rendered hard by the deposit of mineral salts, the resulting material being of great strength and rigidity. The texture may be close and dense like ivory or open and spongy, the difference lying merely in the fact that the one has fewer spaces between the solid particles than the other. There is usually a hard, compact layer on the exterior of the bone, as that is where the greatest cross-strain comes, especially in the long bones, while within is the cancellous or spongy tissue, which gives lightness to the bone and is capable of withstanding enormous pressure, though it can bear little cross-strain.
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Fig. 4.—Cross-section of compact bone tissue. (After Sharpey.)
Structure of Bone.—The hard substance in bone is always arranged in lamellæ or bundles of bony fibers, which in cancellous tissue meet to form a kind of lattice-*work, while in the dense tissue they are generally arranged in rings about the Haversian canals, channels through which the blood-vessels pass through the bone longitudinally. Between the lamellæ are spaces called lacunæ, in which lie branched cells, the spaces being connected with each other and with the Haversian canals by numerous tiny canals or canaliculi, by which
