This section is from the book "The Manufacture Of Boots And Shoes: Being A Modern Treatise Of All The Processes Of Making And Manufacturing Footgear", by F. Y. Golding. Also available from Amazon: The Manufacture Of Boots And Shoes.
The surfaces of the bones that meet at joints are tipped with gristle, or a layer of cartilage fixed firmly to the bone. Between the cartilages is found a synovial membrane. This is a kind of bag containing a small quantity of lubricating fluid, termed synovia. In health there is only sufficient to enable the surfaces, which are smooth, to glide easily over each other.
The Ankle-joint is formed by the astragalus and the bones of the leg. Its use is to allow the foot to be flexed or extended, raised or depressed. It is a strong hinge-joint, and scarcely any lateral movement takes place. In fact, when the foot is at right angles to the tibia, no lateral movement is permitted, and only when the foot is bent does a slight lateral movement take place between the astragalus and the tibia, and this owing to the astragalus being narrower behind than in front. The tibia and fibula are united by a ligament, and when the foot is fully flexed, the larger front of the huckle-bone opens the tibia and fibula, and so exercises the ligament. The opening is not directly forwards, but slightly outwards, so that when feet are fully extended they incline to each other, and when flexed they decline. If a person stands on tiptoe, the ankles separate. If the position be changed to the heels, and the foot extended, the great toe of each foot will approach; and if flexed, they will diverge. This is an important fact to bear in mind in -designing-lasts for various-height heels, and such work as running shoes.
A second joint in the foot is between the astragalus and the os calcis. It is brought into play when the foot is moved from side to side. A third joint is between the astragalus and os calcis, and the scaphoid and cuboid, and allows the foot to be raised on the inner side and depressed on the outer side.
Ligaments are bands of flexible, tough, inextensible, somewhat silvery-looking, fibrous tissue; their office being to limit the movements of a joint. They often work alternately with the muscles, being of mutual advantage in preserving and developing strength in each other. The arch of the foot owes its shape largely to their aid.
The Plantar Ligament connects the os calcis with the metatarsal, and is often likened to the tie-beam of a roof, and has been supposed to be the means of maintaining the arch, although it would be more correct to say it assists the muscles in forming the arch, A, Fig. 8.
The Calcaneo-Scaphoid Ligament is another important ligament of the foot. It is composed of elastic tissue and supports the huckle-bone. The quality of elasticity is not common to other ligaments, and its usefulness consists in allowing the astragalus to descend a little when weight is borne upon it, and after it is relieved of weight it forces the key-stone again to its normal position, thus giving, among other provisions, elasticity to the step. Improper boots preventing the usage of the ligaments will cause them to deteriorate, and "weak ankle" and" flat foot" are exhibited. This ligament in use adds its share to the elongation of the foot (see B, Fig. 8). When the toes of the foot are turned out, the scaphoid bone is lowered, and the ligament uniting it to the os calcis is relaxed, so the astragalus lowers, and vice versa if the toes of the foot are turned inwards.
There are other ligaments in the foot, such as the inter-osseus (Fig. 9) between the astragalus and the os calcis, and those between the cuneiform bones; also the annular ligament that binds the tendons to the bend or curve in front of the foot. The plantar fascia is a very dense sheet of fibrous tissue fastened to the heel behind and spread over the sole, and fixed to the bone at the ball of the foot. It should be borne in mind that constant pressure or strain on a ligament causes wasting, but that intermittent pressure promotes growth.