thickest and strongest tendon in the body is the tendon of Achilles,
which connects the great muscles in the calf of the leg with the heel bone
(sec. 49).

When muscles contract forcibly, they pull upon the tendons which transmit
the movement to the bones to which they are attached. Tendons may be
compared to ropes or cords which, when pulled, are made to act upon
distant objects to which one end is fastened. Sometimes the tendon runs
down the middle of a muscle, and the fibers run obliquely into it, the
tendon resembling the quill in a feather. Again, tendons are spread out in
a flat layer on the surface of muscles, in which case they are called
aponeuroses. Sometimes a tendon is found in the middle of a muscle as well
as at each end of it.

[Illustration: Fig. 34.--The Biceps Muscle dissected to show its Tendons.]

72. Synovial Sheaths and Sacs. The rapid movement of the tendons
over bony surfaces and prominences would soon produce an undue amount of
heat and friction unless some means existed to make the motion as easy as
possible. This is supplied by sheaths which form a double lining around
the tendons. The opposed surfaces are lined with synovial
membrane,[11] the secretion from which oils the sheaths in which the
tendons move.

Little closed sacs, called synovial sacs or bursæ, similarly lined
and containing fluid, are also found in special places between two
surfaces where much motion is required. There are two of these bursæ near
the patella, one superficial, just under the skin; the other deep beneath
the bone (Fig. 29). Without these, the constant motion of the knee-pan and
its tendons in walking would produce undue friction and heat and