108 



ANIMAL MECHANICS. 



and $ denote the angle ab V, made by its fibres with the par- 

 allel bars, the resultant force of the penniform muscle will be 



R = 2F cos 0. 



C25) 



Many muscles, when dissected, are found to contain parallel 

 sheets of fascia, resembling the bars AA, BB, and the mus- 

 cular fibres take origin from these sheets of fascia, forming 

 true penniform muscles, which, taken together, make up the 

 entire complex muscle. Thus, the deltoid muscle is consi- 

 dered by Albinus to be composed of seven distinct pennate 

 muscles. In the biceps humeri of the Ungulates I have found, 

 also, a penniform arrangement of the fibres with reference to 

 fascial sheets that run along the whole course of the muscle. 



Let ^IXand BX (Fig. 25) 

 represent two fibres of a pen- 

 nate muscle, and let I be the 

 length of each ; during their 

 contraction, they are each 

 shortened by a length Sl f and 

 the point X is moved to the 

 points; draw xp perpendi- 

 cular to BX, then Xp is equal 

 to $1, and the distance through 

 which X is moved is 



Xx - Bl sec (p. 



Multiplying this distance by the resultant force (25), in the 

 direction XY y we have — ■ 



Work done by the contraction of a pennate muscle 



= 2.Fcos <p sec $81 = iFdl. (^6) 



And, since zFSl is the work inherent in the muscular fibres, 

 if arranged in a prismatic manner, we conclude that there is 



