of blood vessels is quite small. The tissues are also supplied with lymphatics and nerves.
Cartilage.—Cartilage or gristle is a tough and dense tissue possessing a certain degree of flexibility and high elasticity. It is found where a certain amount of flexibility is required but where a fixed shape must be retained, e.g. in the trachea which must always be kept open or in the external ear or pinna which owes its typical and permanent shape to the presence of cartilage. It is largely associated with the bones in the formation of the skeleton. The tissue consists of a number of cells embedded in a solid matrix or ground substance. Three varieties are distinguished according to the nature of the matrix. Thus if the matrix is homogeneous in structure the cartilage is termed hyaline. Two other forms occur in which fibrous tissue is embedded in the cartilage matrix. They are therefore termed fibro-cartilages and if the fibres are of the white variety the cartilage is called white fibro-cartilage, if of the yellow elastic form, elastic cartilage.
Hyaline Cartilage (fig. 5).—This consists of a number of rounded cells enclosed within a homogeneous matrix. The cells possess an oval nucleus and a granular, often vacuolated cell-body.
Fig. 5.—Hyaline Cartilage.
Homogeneous matrix interspersed
with groups of cells
whose arrangement shows their
development by division of the
The number of cells present varies considerably in different specimens. In freshly formed cartilage the cells are numerous, the amount of matrix separating them being small. Cartilage grows by a deposition of new matrix by the cartilage cells which thus become more and more separated from one another. After a time the cells divide and subsequently become parted from one another by deposition of fresh matrix between them. The cells are often to be seen in groups of two, three or four cells, indicating the common origin of each group from a parent cell. Towards the surface of the cartilage the cells are often modified in shape tending to become flattened in a direction parallel to the surface. Some of the cells near the surface of a piece of cartilage may be branched, appearing as a transition form between connective tissue corpuscles and typical cartilage cells. This is particularly the case at points where tendon or ligaments are attached. There may often be a deposit of lime salts in the matrix of hyaline cartilage especially in old animals or in the deeper layers of articular cartilage where it is attached to bone. A similar deposit of lime salts is well marked in the superficial parts of the skeleton of the cartilaginous fishes. In the development of animals possessing a bony skeleton, the skeleton is first laid down as hyaline cartilage which subsequently becomes gradually removed, bone being deposited in its place. In the adult, hyaline cartilage is found at the ends of the long bones (articular cartilage), uniting the bony ribs to the sternum (costal cartilage), and forming the cartilages of the nose, trachea and bronchi, &c. This as well as the other forms of cartilage are non-vascular so that the cells must gain their food-stuffs and get rid of their waste products by a process of diffusion through the matrix, a process which must of necessity be slow.
White Fibro-Cartilage.—This is a variety of cartilage in which numerous white fibres ramify in all directions through the matrix (fig. 6). The cells lie separate and not in groups, and the amount of matrix between is commonly small. The white fibres may run in all directions or may chiefly run in one direction only. Under the microscope the tissue closely resembles a dense white fibrous tissue, only the cells enclosed in it are cartilage cells and not connective tissue cells. Owing to the presence of so much fibrous tissue this variety of cartilage is very much tougher than hyaline cartilage and less flexible. It is found in places which have to withstand a considerable amount of compression but where a less rigid structure than bone is demanded. Thus it is found forming the intervertebral disks, the interarticular cartilages, or at the edges of joint surfaces to deepen the surface.
Fig. 6.—White fibro-cartilage of intervertebral disk, with typical
cartilage cells, matrix characterized by presence of many white
Elastic Fibro-Cartilage.—In this variety the matrix is permeated by a complex and well-defined meshwork of elastic fibres (fig. 7). The size of the fibres varies considerably in different specimens. It is found in parts which have to retain a permanent shape but where a considerable amount of flexibility is requisite, as in the pinna of the ear, the epiglottis, the cartilage of the Eustachian tube, &c.
Fig. 7.—Elastic fibro-cartilage
of Epiglottis. Abundant cartilage
cells in a matrix containing many
branching elastic fibres.
Bone.—Bone is a connective tissue in which a considerable amount of mineral matter is deposited in the intercellular matrix whereby it acquires a dense and rigid consistency. If bone be incinerated so that the organic matter is burnt away, a residue of mineral matter is left. This consists chiefly of calcium phosphate, and amounts to as much as two-thirds of the weight of the original bone. If, on the other hand, bone be macerated in hydrochloric or nitric acid for a time the calcium phosphate is dissolved, leaving the organic matter practically unaffected and still showing the microscopic structure of bone. Hence it follows that the organic matrix is uniformly impregnated with the calcium salts.
According to its naked-eye appearance bone is distinguished as being either compact or cancellated. The former is dense like ivory and forms the outer surface of all bones. The whole of the shaft of a long bone is composed of this compact form. Cancellated bone has a spongy structure and contains large interspaces filled with a fatty tissue rich in blood vessels. This form of bone tissue is found forming the interior of most bones, especially the heads of the long bones, the interior of the ribs, &c. The cavity of the shaft of a long bone is filled, just as in the case of the smaller cavities in cancellated bone, with a fatty tissue, the Bone Marrow (see below).
The histological structure of bone may be made out from a piece of dried bone which has been ground down between grinding stones until it is sufficiently thin for microscopic purposes. If such a section be prepared from a thin transverse slice of a long bone the appearance pictured in fig. 8 will be seen. The section comprises a number of circular units bound into a compact whole by intervening material showing in the main the same structural details. Each of these circular structures is termed an Haversian system. In the centre of each is seen a dark area, the Haversian canal, around which the bone matrix is deposited in the form of a number of concentric laminae. Enclosed between the laminae are a number of small spaces also appearing black in this preparation. These are the bone lacunae and spreading away from them in directions generally transverse to the laminae are seen a large number of fine branching lines—the canaliculi. All parts of a preparation such as this which appear dark in reality represent spaces in the bone matrix. In the course of the preparation of the