CHAPTER IV. 



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 ai-e 

 brought about. A muscle consists of a fleshy belly, which is 

 usually attached at each end by means of tendons to some hard 

 part, very commonly to bone. Motion is effected by the muscle 

 contracting, i.e., 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 m.ovable 

 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, i.e., those 

 which are under the control of the wiU, as the muscles of the 

 arm : and (2) involuntary muscles, i.e., 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 a day or more. When cleaning a muscle be careful to 

 put it on the stretch, and to dissect along, and not across its 

 fibres : define the origin and insertion of the muscle very clearly, 

 and test its action by pulling it gently with the fwce/ps in the 

 direction of its fibres. Always have the skeleton in front of you 

 so as to see accurately the origins and insertions of the muscles. 

 In the following description sorrmof the smaller musdes, especially 

 in the head, are omitted. 



