60 THE MUSCULAR SYSTEM OF THE FROG. 
Chap. V—THE MUSCULAR SYSTEM OF THE FROG. 
The muscles, or flesh, are the direct means by which the 
various movements of the body and of its several parts are 
brought about. A muscle usually consists of a fleshy belly which 
is attached at each end by means of tendons to some hard part, 
very commonly to bone. Motion is effected by the muscle 
contracting, 7.c., shortening, and so bringing its two ends, and 
consequently the parts to which the ends are attached, nearer: 
together. Of the two attachments of a muscle one is usually 
to a more fixed and central part, the other to a more moveable - 
and peripheral part : the former attachment is called the origin 
of the muscle, the latter its insertion. 
Muscles are of two kinds: (1) voluntary muscles, 2.¢., those. 
which are under the control of the will, as the muscles of the 
arm: and (2) involuntary muscles, 2.¢c., those-over which the 
will has no direct control, as the muscles of the heart .and 
bloodvessels, or of the alimentary canal. 
Voluntary muscles, which are the only ones dealt with in this 
chapter, are usually attached at both ends to bone ; but one or 
other end, or both, may be attached to aponeuroses, strong 
connective tissue membranes, which ensheath the muscles and 
other parts, and separate them from one another. 
For the dissection of the muscles, take a frog that has been in spirit 
for aday or more. When cleaning a muscle be careful to put it on 
the stretch, and to dissect along and not cross its fibres: define 
the origin and insertion of the muscle very clearly, and test its 
action by pulling it gently with the forceps in the direction of 
ats fibres. Always have the skeleton in front of you so as to see 
accurately the origins and insertions of the muscles. 
A. Muscles of Trunk. 
1. Muscles of ventral body wall. 
Pin out the frog on its back, remove the skin, and clean the 
muscles. | 
