ANIMAL MECHANICS. 



1(57 



draw the bars together. If (j> denote the angle abH, the ho 

 rizontal and vertical forces exerted by the muscle will be 



// = F cos $ 



V = F sin <j) 



(24) 



The intercostal and rhomboid muscles afford good exam- 

 ples of the Bhomboidal type of muscle. 



Penniform 

 Muscles. — The 

 structure of pen- 

 niform muscles 

 is illustrated by 

 Fig. 24, in which 

 two bars, A A, 

 BB, parallel to 

 each other, are 

 connected by 

 two sets of par- 

 allel fibres, with 

 a third bar, CC, 

 parallel to the 

 other bars. It 

 is a combination 

 of two rhom- 

 boidal muscles, 

 which give a 

 vertical result- 

 ant in the di- 

 rection cv. 



If F denote, 

 as before, the 

 resultant force 

 of each rhom- 

 boidal muscle, 



