180 



ANIMAL MECHANICS. 



that the effect of the whole triangular muscle is the same as 

 that of a single prismatic muscle passing from the tuber ischii 

 at the top of the fibula. 



Another remarkable example of triangular muscle is found 

 in the anterior fin muscle of the Angel Shark ; this muscle 

 consists of a number of distinct muscular bundles, each be- 

 longing to a single ray of the fin, and forming altogether a 

 triangular muscle, whose sides measured 5 in. and in. re- 

 spectively, the base measuring in a straight line 7 inches ; 

 the muscle itself is not terminated by this base, but has a 

 curved base, as shown in the Fig. 29. In this case, as in the 



0 



Fig. 29. 



last, the bisector OX, of the angle AOB, will be the direc- 

 tion of the resultant of the radiating fibres, whatever be the 

 shape of the curve that forms the base of insertion of the 

 muscular fibres. In the Angel Shark I found 20 distinct 

 bundles lying between OB and OX, and 30 bundles lying 

 between OA and OX; but the first bundles were 50 per cent, 

 thicker than the second, so that the muscular force was dis- 

 tributed equally through equal angles round the point 0. 



Let us suppose the radiating fibres of a triangular muscle 

 to be so arranged that equal angles will contain equal num- 

 bers of muscular fibres ; and let it be required to calculate 

 the work done by the contraction of such a muscle, and 



