ANATOMY OF THE HOUSE. 
ISO 
INTERCOSTALES EXTERNI. 
IS umber. —Seventeen on each side. 
Situation. —Filling up the intercostal spaces, outwardly: 
reaching from the articulations of the ribs with the transverse 
processes as far as the commencement of their cartilages. 
Attachment. —Anteriorly, to the external edge of the posterior 
border of one rib ; posteriorly, to the internal edge of the anterior 
border of the rib immediately behind the other. 
Relations .—On the outer side, with the serratus, pectoral mus¬ 
cles, obliquus externus, and transversalis and superficialis costa- 
rum ; on the inner side, with the intercostales interni; supe¬ 
riorly, with the levatores costarum. 
Direction. —Oblique: from before, backward ; and from above, 
downward. 
Structure. —Thick layers of united fleshy fibres included be¬ 
tween broad aponeurotic slips of tendon, and in places displaying 
tendinous intersections. 
INTERCOSTALES INTERNI. 
Number. —The same as the preceding. 
Situation. —Within the intercostal spaces, reaching from the 
angles of the ribs to the sternum. 
Attachment. —To the antero-internal edge of the border of 
one rib ; to the postero-internal edge of the rib immediately 
before it. 
Relations. —On the outer side, with the preceding muscles, 
and the intercostal vessels and nerves; on the inner side, with 
the pleura. 
Direction. —Oblique: but the reverse of that of the former 
muscles; 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 ; 
and 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 aponueroses, 
which separate them from the others. 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 
calibre of the cavity of the chest. 
[To be continued.] 
