EFFECTS OF WITHDRAWAL OF. NERVOUS POWER. 449 
the wasting-away of the muscles which otherwise takes place ; 
and thus we see that the preservation of this peculiar property 
is dependent upon the due nutrition of the muscle, whilst the 
loss of the property results from its want of nutrition,—as we 
find to he the case in regard to other tissues. Further, the 
activity of the nutrition of muscles depends in great part 
upon the use that is made of them ; and thus we find that 
any set of muscles in continual employment undergoes a great 
increase in size and vigour; whilst those that are disused, 
even though their nervous connexions remain entire, lose 
their firmness and diminish in bulk, until, if the inaction be 
continued long enough, almost all trace of proper muscular 
substance disappears, and the contractility of the part is lost. 
588. But a muscle may be palsied by some change taking 
place in the central organs, which shall prevent the nervous 
influence from being excited there. Thus by an effusion of 
blood in a certain part of the brain, the arm, leg, or the whole 
of one side may be paralysed to the influence of the will. 
But the muscles which are thus withdrawn from the power of 
the will, may sometimes be moved by an emotional or instinctive 
impulse, or by reflex action; their connexion with the parts of 
the nervous centres, in which these actions respectively origi¬ 
nate, remaining unimpaired (Chap. x.). Thus a completely 
paralytic arm has been seen to be violently shaken, when the 
emotions of the patient were strongly excited by the approach 
of a friend. The muscles of the shoulder, in a case of com¬ 
plete paralysis of one side, were called into contraction in the 
reflex movement of yawning (§ 341). And the muscles of 
the legs, when their communication with the brain,-—and 
consequently the control of the will over them,—has been 
completely cut off, have been made to act energetically when 
the feet were tickled, although the patient was not conscious 
either of the irritation or of the motion. When the muscles 
are thus aroused to occasional activity, their nutrition is not 
so much impaired, and their contractility does not depart 
nearly as completely as when they are thrown into entire 
disuse by division of their nerves. 
589. Muscles are commonly divided into voluntary and 
involuntary , according as they act in obedience to the will, 
or are not under its dominion. But this is not a correct 
division ; since, whilst nearly all the muscles of the body, are 
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