264 OBSERVATIONS 



for their motion in refpiration. Confidently with this view, a* 

 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 I have before demonftrated, perform more extenfive motion 

 than the latter, even where both are of the fame length, they 

 mud have a flill greater effect, where the two kinds of mufcles 

 are confined between the fame parallels. 



Thus, 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 ihortening itfelf one-fifth part of its length ; it 

 appears from the demonftration, that the oblique mufcle can 

 move the rib through a fpace five times greater than 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 intercoflal 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. 



The only point remaining to be explained, is, why nature 

 hath formed two layers of intercoflal mufcles decufTating each 

 other. 



The purpofe of this, I apprehend, is to render the motion of 

 the rib upwards as direct as poffible, and to prevent it from be- 

 ing drawn or prefTed forwards upon the fternum, or backwards 

 upon the vertebrae, fo much as, by its friction, to interrupt the 

 freedom of its motion. 



3 Upon 



