The pelvis is the posterior part of the trunk. It incloses the pelvic
cavity (Cavum pelvis), which communicates in front with the abdominal
cavity, the line of demarcation being the pelvic brim or terminal line.
The =dorsal wall= or roof is formed by the sacrum and first three
coccygeal vertebræ. The =lateral walls= are formed by the parts of the
ilia behind the ilio-pectineal lines and the sacro-sciatic ligaments.
The =ventral wall= or floor is formed by the pubic and ischial bones.
The boundary of the =outlet= is formed by the third coccygeal vertebra
dorsally, the ischial arch ventrally, and the posterior edges of the
sacro-sciatic ligaments and the semimembranosus muscles laterally. The
outlet is closed by the perineal fascia; this consists of superficial
and deep layers, which are attached around the margin of the outlet and
centrally to the organs at the outlet—the anus and its muscles, the
vulva (in the female), and the root of the penis (in the male).
The cavity contains the rectum, parts of the internal generative and
urinary organs, some fœtal remnants, muscles, vessels, and nerves. It is
lined by the fascia pelvis, and in part by the peritoneum.
The =pelvic peritoneum= is continuous in front with that of the abdomen.
It lines the cavity as far back as the third or fourth sacral vertebra
in the horse, where it is reflected on to the viscera, and from one
organ to another. We may therefore distinguish an anterior, peritoneal,
and a posterior, retroperitoneal part of the cavity. Along the
mid-dorsal line it forms a continuation of the colic mesentery, the
=mesorectum=, which attaches the first or peritoneal part of the rectum
to the roof. In animals in fair condition a considerable quantity of
subperitoneal and retroperitoneal fat is found on the walls and in the
various interstices.
[Illustration:
FIG. 256.—DIAGRAM OF SAGITTAL SECTION OF MALE PELVIS TO SHOW
DISPOSITION OF PERITONEUM.
_a_, Pouch between rectum and roof of cavity, continuous laterally
with _b_, recto-genital pouch; _c_, vesico-genital pouch; _d_, pouch
below bladder and its lateral ligaments. The lateral line of
reflection of the peritoneum is dotted. The area of rectum covered
by peritoneum varies widely. When the rectum is empty, the
reflection dorsally may be at the posterior end of the sacrum; when
the rectum is very full, the reflection may occur a short distance
behind the promontory.
]
[Illustration:
FIG. 257.—SCHEMATIC CROSS-SECTIONS TO SHOW ARRANGEMENT OF PELVIC
PERITONEUM OF HORSE: A, IN MALE; B, IN FEMALE.
_A_: _a_, _b_, Recto-genital pouch, _c_, _c_, vesico-genital pouch;
_d_, _d_, pouch below bladder and its lateral ligaments; _1_,
mesorectum; _2_, _2_, urogenital fold; _3_, _3_, lateral, _4_,
median ligaments of bladder; _v. d._, vas deferens; _u. m._, uterus
masculinus. _B_: _a_, _b_, recto-genital pouch; _c_, _c_,
vesico-genital pouch; _d_, _d_, pouch below bladder and its lateral
ligaments; _1_, mesorectum; _2_, _2_, broad ligaments of uterus;
_3_, _3_, lateral, _4_, median ligaments of bladder.
]
In the male the general disposition of the peritoneum here is as
follows. If traced along the dorsal wall, it is reflected at the third
or fourth sacral vertebra on to the rectum, forming the visceral
peritoneum for the first part of that tube. Laterally it is reflected in
a similar fashion. If the rectum be raised, it will be seen that the
peritoneum passes from its ventral surface and forms a transverse fold
which lies on the dorsal surface of the bladder (Fig. 272). This is the
=urogenital fold= (Plica urogenitalis). Its concave free edge passes on
either side into the inguinal canal. The ventral layer of this fold is
reflected on to the dorsal surface of the bladder near its neck. Thus
there is formed a pouch between the rectum and bladder—the
=recto-vesical pouch= (Excavatio recto-vesicalis), which is partially
subdivided by the urogenital fold into recto-genital and vesico-genital
cavities. The fold contains the vasa deferentia, part of the vesiculæ
seminales, and the uterus masculinus (a fœtal remnant). The space on
either side of the rectum is occupied by coils of the small colon and
the pelvic flexure of the great colon usually. If the bladder is now
raised, it is seen that the peritoneum passes from its ventral surface
on to the pelvic floor, forming a median fold, the so-called =middle
ligament= (Plica umbilicalis media). It also forms on each side a
lateral fold, the =lateral ligament= (Plica umbilicalis lateralis),
which contains in its edge the so-called =round ligament= (Ligamentum
teres)—the partially occluded umbilical artery, which is a large vessel
in the fœtus.
[Illustration:
FIG. 258.—STOMACH OF HORSE, PARIETAL SURFACE, WITH FIRST PART OF
DUODENUM.
Fixed _in situ_ when full but not distended. The larger branches of
the anterior gastric artery with two satellite veins are shown.
]
In the female the arrangement is modified by the presence of the uterus;
the urogenital fold is very large, so as to inclose the uterus and a
small part of the vagina. It forms two extensive folds, the =broad
ligaments of the uterus= (Ligamenta lata uteri), which attach that organ
to the sides of the pelvic cavity and the lumbar part of the abdominal
wall (Fig. 271). It thus divides the recto-vesical pouch completely into
dorsal and ventral compartments—the =recto-genital pouch= (Excavatio
recto-uterina), and the =vesico-genital= pouch (Excavatio
vesico-uterina).
Further details will be given in the description of the pelvic viscera.