s of the short ones. The points or regions where the contact
occurs are denominated the articular surface, which assumes from this
circumstance a considerable variety of aspect and form, being in one
case comparatively flat and another elevated; or as forming a protruding
head or knob, with a distinct convexity; and again presenting a
corresponding depression or cavity, accurately adapted to complete, by
their coaptation, the ball-and-socket joint. The articulation of the arm
and shoulder is an example of the first kind, while that of the hip with
the thigh bone is a perfect exhibition of the latter.

The structure whose office it is to retain the articulating surfaces in
place is the ligament. This is usually a white, fibrous, inelastic
tissue; sometimes, however, it is elastic in character and yellowish. In
some instances it is funicular shaped or corded, serving to bind more
firmly together the bones to which its extremities are attached; in
others it consists of a broad membrane, wholly or partially surrounding
the broad articulations, and calculated rather for the protection of the
cavity from intrusion by the air than for other security. This latter
form, known as capsular, is usually found in connection with joints
which possess a free and extended movement. The capsular and funicular
ligaments are sometimes associated, the capsular appearing as a
membranous sac wholly or partially inclosing the joint, the funicular,
here known as an interarticular ligament, occupying the interior, and
thus securing the union of the several bones more firmly and effectively
than would be possible for the capsular ligament unassisted.

The universal need which pertains to all mechanical contrivances of
motion has not been forgotten while providing for the perfect working of
the interesting piece of living machinery which performs the function of
locomotion, as we are contemplating it, and nature has consequently
provided for obviating the evils of attrition and friction and insuring
the easy play and smooth movement of its parts by the establishment of
the secretion of the synovia, the vital lubricant of which we have
before spoken, as a yellow, oily, or rather glairy secretion, which
performs the indispensable office of facilitating the play of the
tendons over the joints and certain given points of the bones. This
fluid is deposited in a containing sac, the lining (serous) membrane of
which forms the secreting organ. This membra