on muscular Motion. 
2 ^ 
affects the right hand. An ingenious practical chemist in London 
has frequently experienced spasms and rigidity in the muscles 
of his fore arms, from affusions of nitric acid over the cuticle 
of the hand and arm. The use of mercury occasionally brings 
on a similar rigidity in the masseter muscles. 
A smaller quantity of blood flows through a muscle during 
the state of contraction, than during the quiescent state, as is 
evinced by the pale colour of red muscles when contracted. 
The retardation of the flow of blood from the veins of the fore 
arm, during venaesection, when the muscles of the limb are kept 
rigid, and the increased flow after alternate relaxations, induces 
the probability, that a temporary retardation of the blood in the 
muscular fibrils takes place during each contraction, and that 
its free course obtains again during the relaxation. This state 
of the vascular system in a contracted muscle, does not, how- 
ever, explain the diminution of its bulk, although it may have 
some influence on the limb of a living animal. 
When muscles are vigorously contracted, their sensibility 
to pain is nearly destroyed ; this means is employed by jug- 
glers for the purpose of suffering pins to be thrust into the 
calf of the leg, and other muscular parts with impunity : it is 
indeed reasonable to expect, a priori, that the sensation, and 
the voluntary influence, cannot pass along the nerves at the 
same time. 
In addition to the influences already enumerated, the human 
muscles are susceptible of changes from extraordinary occur- 
rences of sensible impressions. Long continued attention to 
interesting visible objects, or to audible sensations, are known 
to exhaust the muscular strength : intense thought and anxiety, 
weaken the muscular powers, and the passions of grief and 
E 2 
