APPLICATION OF MECHANICAL PRINCIPLES. 



429 



Fig. 184. 



Action of Intercostals. 



tract as to admit of a close approximation of the ribs ; whilst the 

 oblique muscles will admit of this to a much greater extent. We can, 

 therefore, understand, why the intercostal muscles pass obliquely from 

 one rib to another, as at D and B C, Fig. 184, instead of in a direction 

 perpendicular to the two ribs as at A. 



There are cases, however, in which a straight muscle may pass 

 between two parallel ribs, and carry them through a given space, with 

 less decurtation of fibres, than any oblique muscle, which has the same 

 origin; but is inserted at a greater 

 distance from the centre of motion, 

 and acts through the medium of a 

 longer lever. Moreover, a mus- 

 cle, with a less degree of obliquity, 

 may be so situate as to carry the 

 bones through a given space, with 

 a less decurtation of fibres than 

 any other muscle having the same 

 origin but a much greater de- 

 gree of obliquity. Suppose A B and C D, Fig. 185, to be two 

 parallel ribs, of which A B is movable about A as a centre ; and sup- 

 pose it to be brought 

 by the action of the Fig. 185. 



straight muscle E F, j? O- 



and of the oblique 

 muscles E G and E 

 H, into the position 

 A/. The points of 

 insertion of the mus- 

 cles will now be at a, 

 c, and e, after having 

 traversed the spaces 

 F a, G c, and H e. 

 If we now, from the point E, as a centre, describe the arcs c b and e d; 

 the spaces d H and b G will indicate the degree of decurtation, which 

 the oblique muscles have experienced, and a F that of the straight mus- 

 cle. This figure also shows, that when the muscles change the relative 

 position of any two bones, they at the same time change the direction 

 of their own action, and vary their lever. When the rib A B is brought 

 into the position A/, the muscles E G and E H, by being brought 

 down to c and 6, have assumed the positions E c and E e; and have, con- 

 sequently, changed their length, situation, obliquity, and leverage. 



Again, of the muscles, which are attached to ribs that are parallel, 

 equally movable, and situate at right angles to the spine, those which 

 pass perpendicularly from one rib to the other will act upon each with 

 equal leverage; and each will approach the other with the same velo- 

 city; whilst those that pass obliquely from one to the other, will .make 

 them approach with different velocities; a principle which is strikingly 

 applicable to the intercostal muscles. Let us suppose A B and C D, 

 Fig. 186, to be two parallel ribs, articulated with the spine at A and 

 C, and equally movable on these centres of motion. Let D B repre- 



:o 



Action of Intercostals. 



