208 ANIMAL PHYSIOLOGY. 
Summary of the Physiology of Muscle and Nerve—The move- 
ménts of a muscle are distinguished from those of other forms 
of protoplasm by their marked definiteness and limitation. 
The contraction of a muscle-fiber (cell) results in an increase 
in its short transverse diameter, and a diminution of its long 
diameter, without appreciable change in its total bulk. 
Muscle and nerve are not automatic, but are irritable. 
Though muscle normally receives its stimulus through a nerve, 
it possesses independent irritability. 
Stimuli may be mechanical, chemical, thermal, electrical, and 
in the case of muscle, nervous; and to be effective they must 
be applied suddenly and last for a brief but appreciable time. 
Electrical stimulation, especially, is only effective when 
there is a sudden change in the force or direction of the cur- 
rents. This applies to both muscle and nerve. 
A muscular contraction consists of three phases: the latent 
period, the period of rising, and the period of falling energy, 
or of contraction and relaxation. 
When the phase of relaxation is minimal and that of con- 
traction approaches continuity, a tetanus results. The contrac- 
tions of the muscles in sitw are tetanic, and are accompanied 
by a low sound, evidence in itself of their vibratory character. 
The prolonged contraction of a muscle leads to fatigue; 
owing in part, at least, to the accumulation of waste-products 
within the muscle which depress its energies. 
This is a necessary consequence of the fact that all proto- 
plasmic activity is accompanied by chemical change, and that 
some of these processes result in the formation of products 
which are hurtful and are usually rapidly expelled. 
Muscular contraction is accompanied by chemical changes, 
in which the formation of carbon dioxide, and some substance 
that causes an acid reaction to take the place of an alkaline or 
neutral one. Since free oxygen is not required for the act of 
contraction, but is still used up by a contracting muscle, it may 
be assumed that the oxygen that plays a part in actual con- 
traction is intra-molecular. 
Chemical changes are inseparable from the vital processes 
of all protoplasm, and the phenomena of muscle show that 
they are constantly in operation, but exalted during ordinary 
contraction and that tetanic condition which precedes and 
may end in coagulation of muscle plasma and the formation of 
myosin. The latter is a result of the disorganization of muscle, 
and has points of resemblance to the coagulation of the blood. 
