MUSCULAR CONTRACTION. 
73 
structure ; and we find it dispersed in small quantity through 
the latter in the skin, to which (especially in particular 
regions) it gives a contractility that is manifested under the 
influence of cold or of mental emotions, and thus produces 
that general roughness and rigidity of the surface which is 
known as cutis anserina, or “ goose’s skin.” 
58. Under the influence of certain exciting causes, or 
stimuli (Chap, xn.), striated muscular fibres suddenly and for¬ 
cibly contract. Their two ends approach one another, and their 
striae become closer; but they bulge out in the middle to a 
corresponding degree. This causes a like change in the bundles 
which are made up of these fibres ; and thus the whole muscle, 
when shortened by the drawing together of its two ends, is 
greatly enlarged in diameter, especially towards its middle. 
Of this any one may convince himself, by bending his fore¬ 
arm upon the arm (as when the hand is brought to the 
mouth), and feeling the fleshy mass upon the front of the 
latter. The muscle, in fact, does not in the least degree 
change its own bulk in the act of contraction ; for its enlarge¬ 
ment in diameter is exactly equivalent to the shortening of 
the distance between its extremities. The contraction of a 
muscular fibre is ordinarily followed, after a short interval, 
by its relaxation; of this we have a remarkable illustration in 
the contractions excited by the electric stimulus. But relax¬ 
ation of individual fibres is not incompatible with the con¬ 
tinuance of the state of contraction of the muscle as a whole. 
Bor it appears that when an ordinary muscle is thrown into 
contraction, all its fibres do not usually contract together, but 
only a small part of them; and that, as long as its contraction 
is maintained so as to exert a constant force, a continual in¬ 
terchange is taking place in the action of the fibres by which 
this is kept up—those which have been shortened becoming 
slack, and being replaced (as it w^ere) by others, which pass 
into the contracted state for a time, and then relax again, 
being succeeded by another set. Now as the ends of those 
fibres which are actually in a relaxed condition, are brought 
near together by the contraction of the rest, the fibre is 
thrown out of the straight line, and assumes a wavy or zigzag 
form, which was formerly supposed to be the state of con¬ 
traction, but is now known to be otherwise. This peculiar 
arrangement-gives place to the straight form, either when the 
