Part 41

The =fibula= extends the entire length of the region. It is slender, somewhat twisted, and enlarged at either end. The upper part of the shaft is separated from the tibia by a considerable interosseous space, but the lower part is flattened and closely applied to the tibia. The proximal end is flattened and articulates with the external condyle of the tibia. The distal end is somewhat thicker and forms the external malleolus. It articulates internally with the tibia and the tibial tarsal bone. Externally it bears two tubercles.

The =patella= is long and narrow. The anterior surface is convex in both directions. The articular surface is convex from side to side and slightly concave from above downward.

The =tarsus= comprises seven bones. The tibial tarsal consists of a body, neck, and head, like the bone in man. The body presents a trochlea above for articulation with the tibia and fibula. The posterior surface has three facets for articulation with the fibular tarsal bone. The head is directed a little inward and articulates with the central. The fibular tarsal has a long anterior process or “beak,” but the inner process (sustentaculum) is short. The tuber calcis is grooved from before backward. The central has a concave proximal surface adapted to the head of the tibial tarsal. Its distal surface articulates with the first, second, and third tarsals. It bears two tubercles posteriorly. The first tarsal is flattened and irregularly quadrangular; it articulates above with the central and below with the first metatarsal. The second tarsal is the smallest and is wedge-shaped; it articulates below with the second metatarsal bone. The third tarsal is also wedge-shaped, the base being in front; it articulates with the third metatarsal below. The fourth tarsal is remarkably high, and resembles a quadrangular prism; it articulates with the fibular tarsal above, the fourth and fifth metatarsal below, and the central and third tarsal bones internally. A groove for the tendon of the peroneus longus crosses its outer and posterior surface, and above it are one or two tubercles.

Five =metatarsal bones= are present. The first is commonly very small and has the form of a blunt cone, somewhat compressed laterally. It articulates with the first tarsal and furnishes insertion to the tibialis anterior muscle. In some cases it fuses with the first tarsal; when the first digit is well developed, its metatarsal may resemble the others (except in size) or be reduced in its proximal part to a fibrous band. The other metatarsals are a little longer than the corresponding metacarpals. Their proximal ends are elongated from before backward and have posterior projections, which in the case of the third and fourth usually bear facets for articulation with two small rounded sesamoid bones. In other respects they resemble the metacarpals.

The first digit is often absent. When present, its development varies and it contains one or two phalanges. In other cases the digit is double. The phalanges of the other digits resemble those of the fore limb.

ARTHROLOGY

THE ARTICULATIONS OR JOINTS

An =articulation= or =joint= is formed by the union of two or more bones or cartilages by other tissue. Bone is the fundamental part of most joints; in some cases a bone and a cartilage, or two cartilages, form a joint. The uniting medium is chiefly fibrous tissue or cartilage, or a mixture of these. Union of parts of the skeleton by muscles (Synsarcosis), as in the attachment of the thoracic limb in the horse, will not be considered in this section.

Joints may be classified—(a) anatomically, according to their mode of development, the nature of the uniting media, and the form of the joint surfaces; (b) physiologically, with regard to the amount and kind of movement or the absence of mobility in them; (c) by a combination of the foregoing considerations.

The classification of joints is still in a very unsatisfactory state, and, unfortunately, the same term is used in various senses by different authors. The two main subdivisions proposed by Hepburn are: (1) Those in which the uniting medium is coextensive with the opposed joint surfaces, and in which a direct union of these surfaces is thereby effected. (2) Those in which the uniting medium has undergone interruption in its structural continuity, and in which a cavity of greater or less extent is thus formed in the interior of the joint. This distinction is of considerable importance clinically.

Three chief subdivisions of joints are usually recognized—viz., =synarthroses=, =diarthroses=, and =amphiarthroses=.

SYNARTHROSES

In this group the segments are united by fibrous tissue or cartilage, or a mixture of the two in such a manner as practically to preclude movement; hence they are often termed fixed or immovable joints. There is no joint cavity. Many of these joints are temporary, the uniting medium being invaded by the process of ossification, with a resulting ankylosis or synostosis. The chief classes in this group of joints are as follows:

(1) =Suture.=—This term (Sutura) is applied to those joints in the skull in which the adjacent bones are closely united by fibrous tissue—the sutural ligament. In many cases the edges of the bones have irregular interlocking margins, forming the =sutura serrata=, e. g., the frontal suture. In others the edges are beveled and overlap, forming the =sutura squamosa=, e. g., the parieto-temporal suture. If the edges are plane or slightly roughened, the term =sutura harmonia= is applied to the joint, e. g., the nasal suture.

(2) =Synchondrosis.=—In these the two bones are united by cartilage, e. g., the joint between the basioccipital and the sphenoid bone. Very few of these joints are permanent.

(3) =Symphysis.=—This term is usually limited to a few median joints which connect symmetrical parts of the skeleton, e. g., symphysis pelvis, symphysis mandibulæ. The uniting media are cartilage and fibrous tissue. In some cases a cleft-like rudimentary joint cavity occurs.

(4) =Gomphosis.=—This term is applied to the implantation of the teeth in the alveoli.

DIARTHROSES

These joints are characterized by the presence of a joint cavity and by their mobility. They are often called movable or true joints. The structures which enter into their formation are:

1. The =joint surfaces= (Facies articulares), which are usually more or less expanded. They are in most cases smooth, and vary much in form. They are formed of specially dense bone, which differs histologically from ordinary compact substance. In certain cases (vide Osteology) the surface is interrupted by non-articular cavities known as =synovial fossæ=.

2. The =articular cartilages= (Cartilagines articulares), usually hyaline in type, form a covering over the articular surfaces of the bones. They vary in thickness in different joints; they are thickest on the areas of the greatest pressure, and usually tend to accentuate the curvature of the bone, i. e., on a concave surface the peripheral part is the thickest, while on a convex surface the central part is the thickest. The articular cartilages are non-vascular, very smooth, and have a bluish tinge in the fresh state. They diminish the effects of concussion and greatly reduce friction.

FIG. 138.—DIAGRAM OF SECTION OF DIARTHROSIS.

f.l., Fibrous layer, s.l., synovial layer of joint capsule. The articular cartilages are white, bones dotted, and the joint cavity black in the figure. ]

3. The =joint capsule= (Capsula articularis) is, in its simplest form, a sac, the margins of which are attached around the articulating surfaces. It consists of two layers—an external one, composed of fibrous tissue (Stratum fibrosum), and an internal one, the synovial layer or membrane (Stratum synoviale). The =fibrous layer=, sometimes termed the capsular ligament, is attached either close to the margins of the articular surfaces or at a variable distance from them. Its thickness varies greatly in different situations: in certain places it is extremely thick, and sometimes cartilage or bone develops in it; in other places it is practically absent, the capsule then consisting only of the synovial membrane. Parts of the capsule may undergo thickening and so form ligaments, which are not separable, except artificially, from the rest of the capsule. The =synovial layer= lines the joint cavity except where this is bounded by the articular cartilages; it stops normally at the margin of the latter. It is a thin membrane, and is richly supplied by close networks of vessels and nerves. It frequently forms =folds= (Plicæ synoviales) and =villi= (Villi synoviales), which project into the cavity of the joint. The folds commonly contain pads of fat which fill up interstices and vary in form and position in various phases of movement. The synovial membrane secretes a fluid, the =synovia=, which resembles white-of-egg and lubricates the joint. In many places the membrane forms extra-articular pouches, which facilitate the play of muscles and tendons.

The =joint cavity= (Cavum articulare) is inclosed by the synovial membrane and the articular cartilages. Normally, it is, strictly speaking, only a potential cavity, which contains nothing but a small amount of synovia.

The student must guard against a false conception of the joint cavity which may result from dissections and diagrams, in which an actual cavity of considerable extent appears to exist. A correct idea of the intimate apposition of the parts is best obtained from the study of frozen sections. It is also instructive to examine joints which have been injected so as to distend the capsule fully. It is then seen that the cavity is often of much greater potential extent than one might suppose, and that the capsule is often very irregular in form, i. e., forms a variety of sacculations.

The foregoing are constant and necessary features in all diarthroses. Other structures which enter into the formation of these joints are ligaments, articular discs or menisci, and marginal cartilages.

4. =Ligaments.=—These are strong bands or membranes, usually composed of white, fibrous tissue, which bind the bones together. They are pliable, but practically inelastic. In a few cases, however, e. g., ligamenta flava, ligamentum nuchæ, they are composed of elastic tissue. They may be subdivided, according to position, into =periarticular= and =intraarticular=. Periarticular ligaments are frequently blended with or form part of the fibrous capsule; in other cases they are quite distinct. Strictly speaking, intraarticular ligaments, though within the fibrous capsule, are not in the joint cavity; the synovial membrane is reflected over them. The term seems justifiable, however, on practical grounds. Ligaments which connect directly opposed surfaces of bones are termed interosseous. The special names are based usually on their position, form, and attachments, e. g., lateral or collateral, cruciate, sacro-iliac, etc. In many places muscles, tendons, and thickenings of the fasciæ function as ligaments and increase the security of the joint. Atmospheric pressure and cohesion play a considerable part in keeping the joint surfaces in apposition.