202 ANIMAL PHYSIOLOGY. 
engaged in physical “ training” must forego severe mental ap- 
plication. Nervous energy is required for the muscles, and all 
questions of blood-supply are, though important, subordinate. 
But it would be premature to enter into a full discussion of this 
interesting topic now. 
.The sense of fatigue experienced after prolonged muscular 
action is complex, though there can be no doubt that the nerve- 
centers must be taken into account, since any muscular work 
that, from being unusual, requires closer attention and a more 
direct influence of the will, is well known to be more fatiguing. 
On the other hand, the accumulation of products of fatigue 
doubtless reports itself through the local nervous mechanism. 
Separation of Muscle from the Central Nervous System.— When 
the nerve belonging to a muscle is divided, certain histological 
changes ensue, which may be briefly described as fatty degenera- 
tion, followed by absorption; and when regeneration of the 
nerve-fibers takes place on apposition of the cut ends, a more 
or less complete restoration of the functions of the nerve fol- 
lows, but the exact nature of the process of repair is not yet 
fully agreed upon; it seems, in fact, to vary in different cases 
as to details, though it is likely that, in instances in which 
there is a complete return to the normal functionally, the axis- 
cylinders, at all events, are reproduced. 
The degeneration downward is complete; upward, only to 
the first node of Ranvier. 
Immediately after the section the irritability of the nerve is 
increased, but rapidly disappears, from the center toward the 
periphery (Ritter-Valli law). 
In the mean time the muscle has been suffering. Its irrita- 
bility at first diminishes, then becomes greater than usual to 
shocks from the make or break of the constant current; but 
finally all irritability is lost, and fatty degeneration and disap- 
pearance of true muscular structure complete the history. It 
is theoretically interesting, as well as of practical importance, 
that degeneration may be delayed by the use of the constant 
current, the significance of which we have already endeavored 
to explain. 
The Influence of Temperature.—If a decapitated frog be placed 
in water of the ordinary temperature, and heat be gradually 
applied, the animal does not move (proving that the spinal cord 
alone is not conscious), but the muscles, when 43° to 50° C. is 
reached, contract and become rigid, a condition known as “‘ heat~- 
rigor.” 
