saphena vein.

Q. The sartorius muscle.

R. The sheath of the femoral vessels.

S S. The falciform margin of the saphenous opening,

T. The anterior crural nerve.

U. The pubic part of the fascia lata.

V. The iliac part of the fascia lata.

W. The lower part of the iliacus muscle.

[Illustration: Abdomen and leg, showing blood vessels, muscles
and other internal organs.]
Plate 31

COMMENTARY ON PLATES 32, 33, & 34.

THE DISSECTION OF THE OBLIQUE OR EXTERNAL AND
THE DIRECT OR INTERNAL INGUINAL HERNIAE.

The order in which the herniary bowel takes its investments from the
eight layers of the inguinal region, is precisely the reverse of that
order in which these layers present in the dissection from before
backwards. The innermost layer of the inguinal region is the
peritonaeum, and from this membrane the intestine, when about to
protrude, derives its first covering. This covering constitutes the
hernial sac. Almost all varieties of inguinal herniae are said to be
enveloped in a sac, or elongation of the peritonaeum. This is accounted
as the general rule. The exceptions to the rule are mentioned as
occurring in the following modes: 1st, the caecum and sigmoid flexure of
the colon, which are devoid of mesenteries, and only partially covered
by the peritonaeum, may slip down behind this membrane, and become
hernial; 2nd, the inguinal part of the peritonaeum may suffer rupture,
and allow the intestine to protrude through the opening. When a hernia
occurs under either of these circumstances, it will be found deprived of
a sac.

All the blood vessels and nerves of the abdomen lie external to the
peritonaeum. Those vessels which traverse the abdomen on their way to
the external organs course outside the peritonaeum; and at the places
where they enter the abdominal parietes, the membrane is reflected from
them. This disposition of the peritonaeum in respect to the spermatic
and iliac vessels is exhibited in Plate 32.

The part of the peritonaeum which lines the inguinal parietes does not
(in the normal state of the adult body) exhibit any aperture
corresponding to that named the internal ring. The membrane is in this
place, as elsewhere, continuous throughout, being extended over the
ring, as also over other localities, where subjacent structures may be
in part wanting. It is in these places, where the membrane happens to be
unsupported, that herniae are most liable to occur. And it must be
added, that t