kept beyond their natural stretch. When one of two antagonists is
contracted by the will, the other relaxes in order to give it
play; or at least becomes overpowered by the contraction of the
first. Also when one of such muscles happens to be paralytic, the
other being no longer balanced, or kept on the stretch,
immediately contracts to its natural length, and remains in that
situation. The part to which it is fixed will, of course, be
affected accordingly. If one of the muscles which move the mouth
sideways be destroyed, the other immediately contracting, draws
the mouth awry; and in that situation it remains. The same may be
observed of the leg, the arm, and other parts. Some muscles assist
one another in their action, while others have different actions;
according to their shapes, the course of their fibres, and the
structure of the parts they move.

According to the shape and nature of the bones to be moved, and of
the motions to be performed, the muscles are either long, or
short; slender, or bulky; straight, or round. Where a great motion
is required, as in the leg, or arm, the muscles are long; where a
small motion is necessary, they are short; for a strong motion
they are thick, and for a weak one slender.

Some of the muscles are fastened to, and move bones; others
cartilages, and others again other muscles, as may best suit the
intention to be answered.

With respect to the bones, some are solid and flattened; others
hollow and cylindrical. Every cylindrical bone is hollow, or has a
cavity containing a great number of cells, filled with an oily
marrow. Each of these cells is lined with a fine membrane, which
forms the marrow. On this membrane, the blood vessels are spread,
which enter the bones obliquely, and generally near their middle;
from some branches of these vessels the marrow is secreted; while
others enter the internal substance of the bones for their
nourishment; and the reason why they enter the bones obliquely is,
that they may not weaken them by dividing too many fibres in the
same place.

The bones being made hollow, their strength is greatly increased
without any addition to their weight; for if they had been formed
of the same quantity of matter without any cavities, they would
have been much weaker; their strength to resist breaking
transversely being proportionate to their diameters, as is evident
from mechanics.

All the bones, excepting so much of the teeth as are out of the
socke