MUSCULAR POWER. 123 



contracted beyond a certain extent : and thus does 

 she economize, as much as possible, the expenditure 

 of muscular power, wherever there is a constant 

 call for its exertion. 



The principle which I have just explained, 

 whereby certain advantages result from the ob- 

 liquity of the action of muscular fibres, is applied, 

 not only to the entire muscle, but also to the in- 

 ternal arrangement of its fibres. Thus, we gene- 

 rally find that, in a flat muscle, its upper and 

 under surfaces are covered by a 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 directions on each side ; 

 like the fibres proceeding from the shaft of a pen. 

 A muscle thus constructed has accordingly been 

 termed a penniform 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 (thejfexor pollicis pedis longiis). 

 The arrangement first described, Fig. 40, forms the 

 semi-peunij'orm muscle; an instance of which occurs 

 in the muscle of the leg, which is termed the semi- 

 membranosus. 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. 12) occurs, for example, in the 



