196 



ANIMAL MECHANICS. 



and, therefore 



x y - x r = (xo~ x o). sin e = xx f . sin o 



Hence, when the angle 0 is given, the difference between XI 

 and X' F', bears a constant ratio to the right line XX'. 



An example of the Quadri- 

 lateral muscle, with curved origin Is. 

 and insertion, may be found in 

 the muscle, fig. 40, which re- 

 presents in the Nemestrine Ma- 

 caque the threefold muscle, ten- 

 sor vagince femoris, glutceus maxi- 

 mus, and agitator caudce ; the line 

 Is, II, represents the bony origin 

 of this threefold muscle, and the 

 lower curved line represents its 

 insertion into [the fascia of the 

 thigh; its fibres converge towards 

 a certain point O, the angle be- 

 tween the extreme fibres being 

 40 0 ; and the resultant force of all the fibres lies in the line OX, 

 which bisects the angle between the extreme fibres. 



Quadrilateral muscles, whose lines of origin and insertion 

 lie, strictly, in the same place, are very rare in nature, and 

 the whole group of muscles, called quadrilateral, tends to 

 merge itself in the more complex form of muscle, presently 

 to be described, in which the fibres, although rectilinear, 

 form, when taken together, ruled or skew surfaces. 



Fig. 40. 



Sphincter Muscles, whose Fibres are curvilinear and parallel, 

 lying in the same Plane. 

 In a sphincter muscle, each fibre forms a closed curve, 

 circular, or elliptical in shape ; and the collection of such 



