The matrix of the hyaline cartilage consists of a translucent material called chodrin, which is mainly a mucopolysaccharide with combined sulphate groups. It is a firm, bluish-white, and somewhat elastic material; sometimes fine fibres may be present. The skeleton of the embryo mammal at one stage consists entirely of cartilage. Gradually, most of it is replaced by bone, cartilage remaining in the following locations: xisphisternal cartilage, the sterna ribs, the suprascapular cartilage and thin plates between diaphysis and epiphysis of bones, and the articular cartilages at the ends of bones. In the elasmobranch fishes, the skeleton remains cartilaginous throughout life, e.g. dogfish.
Each plate of hyaline cartilage is covered by a tough fibrous sheath, the perichodrium. The cells which secrete the chondrin, chondroblasts, lie in small spaces called lacunae, in life, they completely fill these lacunae, lying singly, or in twos, fours or eights. The fibres of cartilage are very difficult to demonstrate, and in some cases there do not appear to be any.
In addition to the chondroblasts in their lacunae, the matrix is well packed with bundles of white fibres. Thus the flexibility of this tissue is combined with firmness of the fibres. Fibro-cartilage is found in the intervertebral discs, where its cushioning effect between the vertebrae is important, and in the pubic symphsis. Here it allows of parturition without complete breakage.
This is another variety of cartilage in which yellow fibres form an anastomosing network. The flexibility of this tissue is enhanced by the presence of these elastic fibres, and the tissue readily recovers shape after distortion. Elastic cartilage is present in the epiglottis, the external ear, and supporting the Eustachian tube and the external auditory meatus.
Bone is connective tissue with a large amount of intercellular substance consisting of mineral salts; two –thirds of the weight of a bone is mineral. The mixture of salts consists of 85 percent calcium phosphate, 10 percent calcium carbonate, about 4 percent of magnesium chloride and 1 per cent of calcium fluoride. It is very strong and rigid tissue, providing efficient levers and adequate protection for delicate organs such as the brain and spinal cord.
Each bone is enclosed in tough sheath of fibrous connective tissue, the periosteum. Underneath the periosteum is very dense layer of compact bone, beneath which is a thicker zone of cancellated or spongy bone. In the long bones, a marrow cavity is Present: this is the site of formation of most of the blood corpuscles compact bone consists of longitudinal Haversian systems. Each system comprises a central Haversian canal surrounded by successive lamellae of salts. In life, the canal contains an artery, a vein, a lymphatic vessels, a nerve and some bone cells or osteocytes. All are packed in with areolar connective tissue. At intervals among the lamellae are ostocytes lying I lacunae. From the osteocytes, fine protoplasmic connexions ramify throughout the matrix in minute canaliculi, in longitudinal section. it can be seen that the Haversian canals are connected by the transverse Volkmann’s canals.