116 MUSCULAR SYSTEM. 



Direction. — Oblique: but the reverse of that of the former 

 nmscles ; viz. from behind, forwards; and from below, upwards : 

 in fact, their fibres decussate each other, like the strokes of an X. 



Structure. — They are thinner than the external intercostals ; 

 but are thicker in substance, themselves, near the sternum than 

 in the vicinity of the spine: the reverse being the case with the 

 external muscles. They are covered with very thin aponeuroses, 

 which separate them from the externi. They are entirely fleshy 

 in substance ; but paler and more delicate in their texture than 

 the former. 



Action. — The intercostal muscles (both external and internal) 

 act upon the ribs to which they are attached, posteriorly : they 

 pull the costal arches forwards, which has a tendency to throw 

 them at the same time outwards ; and thus becomes enlarged the 

 caliber of the cavity of the chest. 



4.— STERNAL REGION. 



Comprising some small muscular slips in the vicinity of and 

 attached to the sternum. 



LATERALIS STERN I. — Costo-stenialis. 



Situation. — Upon the antero-inferior part of the outer surface 

 of the thorax, bordering upon the sternum. 



Figure. — Long, flat, thin: trapezoid. 



Attachment. — Anteriorly, to the postero-inferior half of the first 

 rib : posteriorly, to the superior three-fourths of the cartilage of the 

 third rib; whence its aponeurosis is extended to the cartilages of 

 the fourth and fifth, and also to the sternum. 



Relations. — Externally and inferiorly, with the pectoral muscles; 

 superiorly, with the serratus ; internally, with the second rib and 

 intercostal muscles. 



Direction. — Oblique ; from above downwards and from before 

 backwards. 



Structure. — Its anterior attachment is by means of a short flat 

 tendon ; its posterior is entirely aponeurotic. Both its surfaces 

 are partially interspersed with tendinous expansions. 



Action. — To operate in the contraction of the chest by approxi- 

 mating the cartilages of the anterior ribs, and elevating, in some 

 measure, the sternum. 



sTERNo-cosTALEs. — {Extemi et Interni.) 



Several fleshy digitations, interspersed with slips of tendon, 

 running from the cartilages of all the true ribs, excepting the first, 

 to the sternum. Their fibres are taking the same course, and are 

 continuous (in the intervals between the cartilages) with the \w- 

 tercostales. 



