, THE MUSCLES OF THE ABDOMINAL WALL. 403 
crest in front of the rectus muscle. It is directed obliquely upwards, to be inserted 
for a variable distance into the linea alba. The muscle is often absent. 
The rectus abdominis muscle is broad and strap-like, and arises, by an inner 
and an outer head, from the symphysis and crest of the pubis. Expanding as it 
passes upwards, the muscle is inserted into the front of the ensiform cartilage, and 
into the fifth, sixth, and seventh costal cartilages. On its anterior surface are three 
or more transverse tendinous intersections dines transverse), adherent to the sheath 
of the muscle; the lowest opposite the umbilicus, and the highest about the level of 
the costal arch. Enclosed in its sheath, and covered anterior ly by the pyramidalis 
muscle, the rectus conceals the superior and deep epigastric arteries, the terminal 
branches of the lower thoracic nerves (which pierce the muscle to reach the anterior 
abdominal wall), the fold of Douglas, and the fascia transversalis. The inner 
border of the muscle lies alongside the 
linea alba; its outer border is convex, 
and forms the linea semilunaris. 
The sheath of the rectus muscle 
is derived from the aponeuroses of the 
lateral muscles of the abdominal wall, 
which, after enclosing the muscle, give 
rise to the linea alba in the middle line. 
At the linea semilunaris, at the outer 
border of the rectus muscle, the apo- 
neurosis of the obliquus internus splits 
into anterior and posterior layers. The 
anterior layer, joined by the aponeurosis 
of the obliquus externus, passes in front 
of the rectus, and constitutes the 
anterior lamina of the sheath. The 
posterior layer, joined by the apo- 
neurosis of the transversalis muscle, 
passes behind the rectus, and constitutes 
the posterior lamina of its sheath. 
This arrangement obtains in the upper 
three-fourths of the abdominal wall. 
Below the level of the iliac crest the 
sheath of the muscle is deficient 
posteriorly, and a crescentic border, 
the fold of Douglas, marks the lower 
limit of the posterior lamina. In con- 
Fig. 298.—THE SHEATH OF THE RECTUS ABDOMINIS 
: MUSCLE. 
sequence, the rectus in the lower fourth , 
P th ine sai il ves h (I.), In the thoracic wall; (II.), In the upper three- 
of the abdominal wall rests upon the quarters of the abdominal wall ; (III.), In the lower 
fascia transversalis directly. Close ex- fourth of the abdominal wall. 
amination. however usually reveals a A, RECTUS MUSCLE; B, OBLIQUUS EXTERNUS; C, DraA- 
2 i PHRAGM ; D, OBLIQUUS INTERNUS; E, TRANSVER- 
thin layer behind the muscle in contin- SALIS ABDOMINIS. a, Anterior layer of rectus sheath ; 
uity with the fold of Douglas, and mere- b, Fifth costal cartilage ; c, Sixth costal cartilage ; 
au ei 8 iets Reb ae lesan ian gaan Be 
ing below with the fascia transversalis. iiss Oa pBOld cantal ss Bie O., Bosker on) layer vong reotue 
ies thi Ae tl sates a sheath ; 7, Transversalis fascia ; g, Peritoneum ; h, 
n this region tne rectus 18 covere Linea alba. 1, Deep epigastric artery. 
anteriorly by the conjoint tendon, and 
by the aponeurosis of the obliquus externus, which is gradually becoming separate 
from the subjacent aponeurosis. The upper part of the rectus, lying on the chest- 
_ wall, is only covered anteriorly by a single layer of aponeurosis derived from the 
| obliquus externus, which in this situation is giving origin to the pectoralis major 
muscle. 
Inguinal Canal.—These muscles of the abdominal wall form the boundaries 
of the inguinal canal, which transmits the spermatic cord in the male and the 
round ligament in the female through the lower part of the abdominal wall. The 
canal begins at the internal abdominal ring, placed half an inch above Poupart’s 
ligament, and midway between the anterior superior iliac spine and the symphysis 
pubis. It ends at the external abdominal ring, placed above the spine and crest 
