MUSCULAR POWER. 103 



M and N, may be employed; for as they exert forces in the 

 directions p m and p n, there will result a force in the inter- 

 mediate direction p o: and the effect desired will be accom- 

 plished more quickly, and with a smaller extent of contrac- 

 tion in the muscles producing it, than if the same power had 

 been applied by means of a straight muscle in the direction 

 p o.* It is by means of two sets of muscles, acting thus ob- 

 liquely, that the ribs are brought in closer approximation 

 every time that the chest is elevated in breathing. Thu» 

 carefully does nature dispose the muscular fibres so as to 

 obviate the necessity of their being contracted beyond a cer- 

 tain extent: and thus does she economize, as much as possi- 

 ble, the expenditure of muscular power, wherever there is 

 a constant call for its exertion. 



The principle which I have just explained, whereby cer- 

 tain advantages result from the obliquity of the action of 

 muscular fibres, is applied, 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 tex- 

 ture, or thin expansion of ligament or tendon; and that 

 the muscular fibres which are attached to them are direct- 

 ed obliquely from the one to the other, in the manner re- 

 presented 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 muscu- 

 lar fibres pass obliquely from the former to the latter, but in 

 different directions on each side; like the fibres proceedino- 

 from the shaft of a pen. A muscle thus constructed has ac- 

 cordingly been termed di penniform muscle; as is exempli- 

 fied in the straight muscle inserted into the knee-pan (the 

 rectus extensor c?'uris,) and also in the muscle which bends 

 the great toe (the flexor pollicis jjedis lo?igus.) The ar- 

 rangement first described. Fig. 40, forms the se?7ii-pcfi}ii- 

 form muscle; an instance of which occurs in the muscle of 

 the leg, which is termed the semimembranosus. Frequent- 



• See a paper by Dr. Monro, iji the Transactions of the Koyal Society of 

 Edinburgh. Vol. iii. p. 250. 



