264 OBSERVATIONS 
for their motion in refpiration. Confiftently with this view, as 
the ribs are fixed at both ends, fo that they cannot be moved 
backwards and forwards, but are confined to motion upwards 
and downwards, remaining nearly parallel to each other, ob- 
lique mufcles are preferred to ftraight; for if the former can, 
as | have before demonftrated, perform more extenfive motion 
than the latter, even where both are of the fame length, they 
. muft have a ftill greater effect, where the two kinds of mufcles 
are confined between the fame parallels. 
Tuus, fuppofe the direct diftance, or perpendicular drawn 
from one rib to another, to be reprefented by three parts, and 
that the intercoftal mufcle, in confequence of its obliquity, 
meafures five fuch parts, and that each of thefe is capable, when 
in action, of fhortening itfelf one-fifth part of its length; it 
appears from the demonttration, that the oblique mufcle can 
move the rib through a fpace five times greater sca the ftraight 
mufcle can do. 
ON accurate menfuration, I found the length of the inter- 
coftal mufcle to be one inch’and a half, the perpendicular line 
one inch, and the bafe about one and one-eighth inch. Hence, 
calculating on the fuppofition, that the mufcular fibre, in ac- 
tion, fhortens itfelf one-fifth of its length, it will be found, 
that the intercoftal mufcles, in confequence of their obliquity, 
produce a greater motion of the ribs than perpendicular muf- 
cles could have done, nearly in the proportion of 35 to 12. 
Tue only point remaining to be explained, is, why nature 
hath formed two layers of intercoftal mufcles decuffating each 
other. 
Tue purpofe of this, I apprehend, is to render the motion of 
the rib upwards as diredt as poflible, and to prevent it from be- 
ing drawn or prefled forwards upon the fternum, or backwards 
upon the vertebrz, fo much as, by its friction, to sia! the 
freedom of its motion. 
3 Gras 
