On the MUSCLES. 253 
ftraight fibres, or of fibres perpendicular to the ribs, might 
have been difpofed in the fame {pace. 
Tue 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 demonttrate, 
that a pair of oblique mufcles, placed between the fame pa- 
rallels with two ftraight mufcles, perform, with the fame pro- 
portional contraction, more ,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 aéts. Thus, in the annexed figure, (T. 2. fig. 3.) 
if P, L reprefent two parallel lines,and A B reprefents an oblique 
and AC 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 AB, is longer than the perpendicular 
line AC, 
Bur 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 DE 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 dire@tly towards their origins, 
are moved in a diagonal line, between the two mutfcles. 
Tuus, 
