Whenever two bones come in contact with each other during growth processes, a joint is formed. It may be a rigid immovable joint, synarthrosis, as between skull bones, characterized by edge-to-edge sutures that interlock the bones firmly with one another (Fig. 423), or the different embryonic bones may even fuse solidly together, thus obliterating the sutural joints entirely and resulting in a decrease in the total census of bones composing the adult skeleton. An exception to these interlocking sutures of the skull is the overlapping type between the squamosal bone and its neighbors.
A typical joint, however, is one which allows freedom of motion between the bony partners, and by means of skeletal levers makes locomotion possible. These freely movable joints (diarthroses) include hinge joints, as at the elbow and the knee, and ball-and-socket joints which occur at the shoulder and the hip.
A diagram of such a movable joint (Fig. 424) shows that the two neighboring bones do not come into direct contact with each other, but instead their frictional surfaces are faced with yielding cartilage. Between these cartilage surfaces there is inserted a closed sac, or bursa, filled with a lubricating synovial fluid continuously secreted by the cells lining the sac. Outside of these skeletal elements a fibrous air-tight sheath holds the bones together by atmospheric pressure without muscular effort. In mountain climbing a familiar experience is a sense of heaviness at the joints, resulting from the change in atmospheric pressure.
Outside of the joint there is an equipment of muscles controlled by directive nerves. Thus the operation of movable joints involves the combined action of bone, cartilage, connective tissue, muscle, and nerve.
Divisions of the Skeleton
The hard skeletal parts are usually grouped into the axial and the appendicular divisions of the skeleton. The axial skeleton includes the skull, vertebral column, ribs, and sternum, while the appendicular is composed of the pectoral and pelvic girdles as well as the skeleton of the appendages.
As described in Chapter VII, the cartilaginous and bony elements are laid down first as connective tissue. During embryonic development, connective tissue derivatives of the mesenchyme become widely distributed throughout the body, contributing to the body wall, the various organs, and the linings of body cavities, as well as filling in around the myotomes and grouping about the notochord and embryonic neural tube (Fig. 425). A median dorsal septum, separating the dorsal parts of the myotomes of the two sides of the body, extends from the region of the neural tube to the middorsal line, while the ventral ends of the myotomes are separated by a ventral septum. Between successive myotomes are transverse myocommata which are continuous with the median septa and also with a horizontal septum which splits each myotome into a dorsal and a ventral part.