438 
MOVEMENT 
the continued beating of the heart, the rhythmical contrac¬ 
tions of the stomach and intestines, as well as movement in 
other organs. To all these should be added the wide range 
of movements in facial expressions. To accomplish these 
varied motions man uses more than five 
hundred muscles. 
347. Action of a Voluntary Muscle. —• 
By placing the hand on the front of the 
forearm and raising the hand toward the 
shoulder, the muscle under the hand be¬ 
comes shorter, thicker, and firmer. At 
the lower end of this muscle, a strong 
cord can be felt as the forearm is lowered. 
This cor^l is the tendon that attaches the 
muscle to one of the bones of the forearm. 
The upper end of the muscle is covered 
by other muscles, but we know that it is 
attached to the shoulder blade by two tendons. This muscle 
is the biceps, and like all other arm and leg muscles consists 
of a central part, the belly. It has its origin on the shoulder 
blade by two tendons and is inserted on the radius bone 
by one tendon. When a voluntary muscle produces motion, 
the two ends of the muscle are drawn closer together and 
the belly becomes shorter, 
thicker, and firmer. The 
living muscle cells are the 
only parts that undergo 
any change in shape, while Figure 390 . — Involuntary Muscle 
the living cells in the con¬ 
nective tissue sheaths and in the tendons remain unchanged. 
348. Nerve and Blood Supply of Muscles. —The muscles 
are richly supplied with numerous large and minute blood 
vessels and it is the blood that gives the deep red color to 
muscles. In recent years scientists have discovered that 
each muscle fiber is supplied with a fine branch of the main 
Figure 389 . — Vol¬ 
untary Muscle 
Cells. 
Showing how the 
cells are bound to¬ 
gether with connec¬ 
tive tissue. At the 
end of the muscle, 
the cells of the con¬ 
nective tissue form 
the tendon. 
