MUSCULAR POWEK. 133 



liquity of the action of muscular fibres, is ap- 

 plied, not only to the entire muscle, but also to 

 the internal arrangement of its fibres. Thus, we 

 generally find that, in a flat muscle, its upper 

 and under surfaces are covered by a thin sheet of 

 fibrous texture, or thin expansion of ligament or 

 tendon ; and that the muscular fibres which are 

 attached to them are directed obliquely from the 

 one to the other, in the manner represented by 

 the section, Fig. 40. There is frequently a 

 middle tendinous layer interposed between those 

 that are on the surface (as shown in Fig. 41), in 

 which case the muscular fibres pass obliquely 

 from the former to the latter, but in different di- 

 rections on each side ; like the fibres proceeding 

 from the shaft of a pen. A muscle thus con- 

 structed has accordingly been termed a penni- 

 J'orm muscle ; as is exemplified in the straight 

 muscle inserted into the knee-pan (the rectus 

 extensor cruris), and also in the muscle which 

 bends the great toe (the flexor pollicis pedis 

 longiis). The arrangement first described. Fig. 

 40, forms the semi-penniform muscle ; an instance 

 of which occurs in the muscle of the leg, which 

 is termed the semimembranosus. Frequently the 

 structure is rendered still more complex, by 

 the interposition of several tendinous layers 

 among the fleshy fibres. This arrangement, 

 which constitutes a complex muscle, (as shown in 

 Fig. 4*2) occurs, for example, in the Solceus, or 



