IMPORTANCE OF THE MUSCLES 
437 
walking, or in moving the arms, Figure 388; involuntary , 
such as those which move the food along the digestive tract 
or assist in breathing. 
The voluntary muscles consist of many long muscle cells 
(fibers) bound together by connective tissue into a distinct 
bundle. Usually the muscle bundle 
is attached at each end to the bones 
through the tendon of connective 
tissue. A single muscle moves the 
arm in one direction only, and in order 
to lift the arm from the desk to the 
head, for instance, several muscles 
must act together. 
The cells of the involuntary mjiscles 
are unlike the cells of the voluntary 
muscles. Involuntary muscle cells oc- 
. cur in layers in the walls of the diges¬ 
tive tube, blood vessels, the bladder, 
and the like, and they are not under 
the control of the will. 
The muscular tissue of the heart has 
characteristics of both the voluntary 
and involuntary muscles, so that it 
may almost be said to belong to a 
special class. 
346. Importance of the Muscles.— 
When we remember that 40 per cent 
of the weight of the human body is in r inTundlesThich 
the muscles alone and that one fourth is an adaptation that en- 
of the blood is found in these same ables the le & t0 be movbd 
.... J . .in various directions. 
muscles, their importance is appreci¬ 
ated. But this great mass of tissue so richly supplied with 
blood is solely for the purpose of producing movement. In 
addition to the movements that we are accustomed to see, 
such as the motion of the arms, legs, and the head, there is 
