On the MUSCLES. 253 



ftraight fibres, or of fibres perpendicular to the ribs, might 

 have been difpofed in the fame fpace. 



The other purpofes of oblique mufcles, and which had not 

 been perceived by authors, are, 



To perform much more extenfive motions with the fame de- 

 gree of fhortening of the flefhy fibres, than can be performed 

 by ftraight mufcles, or, with a lefs degree of fhortening of the 

 fibres, to perform motions of equal extent. 



I shall now endeavour, in the firft place, to demonftrate, 

 that a pair of oblique mufcles, placed between the fame pa- 

 rallels with two ftraight mufcles, perform, with the fame pro- 

 portional contraction, more k extenfive motions than the ftraight 

 mufcles can do. 



First, As one oblique mufcle, fo fituate, is longer than a 

 ftraight mufcle, if each be fhortened one third, or any other 

 proportional part, it is evident, that the place of the infertion 

 of the mufcle will move through a greater fpace, when the 

 oblique mufcle acts. Thus, in the annexed figure, (T. 2. fig. 3.) 

 if P, L reprefent two parallel lines, and A B reprefents an oblique 

 and A C a ftraight mufcle, it is plain, that when each is fhort- 

 ened one third, and that the place of the infertion is moved di- 

 rectly towards the place of the origin of the mufcle, the mo- 

 tion occafioned by the oblique mufcle will be proportionally as 

 much greater than that produced by the ftraight mufcle, as 

 the hypotenufe, or line A B, is longer than the perpendicular 

 line A C. 



But next, let us fuppofe, (fee T. 2. fig. 4.) that the point A 

 cannot be drawn directly towards the point D or E, on account 

 of the connections of the bones, fuch as the ribs, which the 

 parallel lines PA and D E reprefent; or fuppofe, that fuch bones, 

 when moving, remain parallel to each other ; or let us fuppofe, 

 that two oblique mufcles balance each other, fo that their in- 

 fertions, inftead of being moved directly towards their origins, 

 are moved in a diagonal line, between the two mufcles. 



Thus, 



