O94 THE MUSCULAR SYSTEM. 
a fan-like manner as they descend, to be inserted into the outer surface of the 
ribs posterior to the angles. They le under cover of the longissimus dorsi 
muscle. 
The infra-costales (subcostales) are slips of muscles found on the inner surface 
of the lower ribs near their angles. They are in series with the internal inter- 
costal muscles, but pass over the deep surface of several ribs. 
The triangularis sterni (m. transversus thoracis) occupies the posterior aspect 
of the anterior thoracic wall. It arises from the back of the ensiform cartilage 
and mesosternum as high as the level of the third costal cartilage. From this 
origin its fibres radiate outwards, the lower ones horizontally, the upper ones 
obliquely upwards, to be inserted into the costal cartilages of all the true ribs 
EXTERNAL INTERCOSTAL Sitemouintercostal 
MUSCLE Anterior intercosta 
membrane 
INTERNAL INTER- 
| COSTAL MUSCLE 
Tie : UY, 
Y MY Ze 
ou ale 
OBLIQUUS EXTERNUS 
ABDOMINIS (reflected) REcTUS ABDOMINIS 
(insertion) 
Sheath of the rectus 
abdominis 
Fic. 289.—THE MUSCLES OF THE THORACIC WALL. 
except the first and seventh. The muscle lies against the pericardium and pleura. 
It is separated from the chest-wall by the internal mammary vessels and the 
anterior branches of the intercostal nerves. The muscle is continuous below with 
the transversalis abdominis. 
The diaphragm is the great membranous and muscular partition separating 
the cavities of the thorax and abdomen. It forms a thin lamella arching over 
the liver, stomach, and spleen, with its convex upper surface in contact with the 
pericardium, pleura and chest-wall. It possesses a peripheral origin from the 
sternum, ribs and vertebral column, and an insertion into a central tendon. It 
arises (1) anteriorly from the posterior surface of the ensiform cartilage by two 
slender fleshy slips, directed backwards; (2) laterally, from the deep surface of the 
lower six ribs on each side by fleshy bands which interdigitate with those of the 
transversalis abdominis; (5) posteriorly, from the lumbar vertebra, by the crura, 
