close, and the head of the thigh bone falls away as far as the ligaments
will allow it.

51. Synovial Membrane. A very delicate connective tissue, called the
synovial membrane, lines the capsules of the joints, and covers the
ligaments connected with them. It secretes the _synovia_, or joint oil, a
thick and glairy fluid, like the white of a raw egg, which thoroughly
lubricates the inner surfaces of the joints. Thus the friction and heat
developed by movement are reduced, and every part of a joint is enabled to
act smoothly.

52. Ligaments. The bones are fastened together, held in place, and
their movements controlled, to a certain extent, by bands of various
forms, called ligaments. These are composed mainly of bundles of
white fibrous tissue placed parallel to, or closely interlaced with, one
another, and present a shining, silvery aspect. They extend from one of
the articulating bones to another, strongly supporting the joint, which
they sometimes completely envelope with a kind of cap (Fig. 28). This
prevents the bones from being easily dislocated. It is difficult, for
instance, to separate the two bones in a shoulder or leg of mutton, they
are so firmly held together by tough ligaments.

While ligaments are pliable and flexible, permitting free movement, they
are also wonderfully strong and inextensible. A bone may be broken, or its
end torn off, before its ligaments can be ruptured. The wrist end of the
radius, for instance, is often torn off by force exerted on its ligaments
without their rupture.

The ligaments are so numerous and various and are in some parts so
interwoven with each other, that space does not allow even mention of