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 mother cell.
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 fibres.
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
