Fig. 98.--_a_, ileum; =b=, ascending colon; _c_, cæcum; _d_, position
of ileocolic valve.
Fig. 99.--_a_, cæcum; _b_, colon; _c_, ileum; _d_, longitudinal muscle
layer; _e_, transverse muscle layer; _f_, mucosa; _g_, ileocolic valve
(opened, as when material is passing into the colon).]
At the bottom of the cæcum on its inner surface is seen a collection of
solitary glands forming one of the agminated glands of Peyer, or Peyer’s
patches. The mucous membrane is without villi. It presents a few
considerable elevations, probably solitary glands.
The =rectum= is the terminal portion of the large intestine lying in the
median line close to the dorsal body wall, from which it is suspended by
the short mesorectum. Its structure is like that of the colon. It opens
externally at the anus. The entire large intestine has a length about
one-half that of the animal. At each side of the anus are two large
secreting sacs, the =anal sacs= or =glands=, each about a centimeter in
diameter. These open into the anus one or two millimeters from its
caudal boundary.
=Muscles of the Rectum and Anus.=--Owing to the close interrelation of
the muscles of the rectum and anus with those of the urogenital organs,
all these muscles will be described together at the end of the
description of the urogenital system.
7. =The Liver=, =Pancreas=, and =Spleen=.--=The Liver.= =Hepar.=--The
liver (Figs. 100 and 101) is a large red-brown organ occupying the
cranial part of the abdominal cavity. It is closely applied to the
caudal surface of the diaphragm and extends thence ventrad of the
stomach so as to conceal all but its pyloric end. Owing to the position
of the stomach the larger mass of the liver is on the right side and it
extends somewhat further caudad on this side.
The liver is divided by the dorsoventral suspensory ligament into the
=right= and =left= lobes, and each half is again divided into lobes. On
the left is a small =left median= (_b_) and a larger =left lateral= lobe
(_a_). The left lateral (_a_) extends caudad with a thin edge which
covers the greater part of the ventral surface of the stomach. On the
right there is a large =right median= (or =cystic=) lobe (_c, c′_). Its
cranial surface is dome-shaped and fitted against the right two-thirds
of the caudal surface of the diaphragm. Its ventral edge is thin, its
dorsal edge thick, and its caudal surface marked by a deep dorsoventral
cleft in which lies the gall-bladder (Fig. 101, _f_). Dorsad and caudad
of the cystic lobe is the =right lateral= lobe (_d, d′_), which is
deeply cleft. Its elongated caudal division (_d′_) extends in a point to
the caudal end of the right kidney and is adapted to the medial half of
its ventral surface. Its smaller and more compact cranial division (_d_)
ends ventrally in a thin edge. It lies between the caudal division
(_d′_) and the cystic lobe (_c_), and its dorsal surface is adapted to
the suprarenal body. The =caudate= or =Spigelian= lobe (Fig. 101, _e_)
is an elongated, triangular, pyramidal lobe. It lies in the omental sac
and partly closes the foramen epiploicum (foramen of Winslow). At its
base it is connected with the caudal division of the right lateral lobe
(_d′_).
[Illustration: FIG. 100.--LIVER, CRANIAL SURFACE.
_a_, left lateral lobe; _b_, left median lobe; _c_, right median lobe;
_d_, _d′_, right lateral lobe; _e_, gall-bladder; _f_, opening of
posterior vena cava, with the smaller openings of the hepatic veins.]
[Illustration: FIG. 101.--LIVER TURNED CRANIAD, SHOWING DORSOCAUDAL
SURFACE.
_a_, left lateral lobe; _b_, left median lobe; _c_, _c′_, right median
(or cystic) lobe; _d_, _d′_, cranial and caudal divisions of the right
lateral lobe; _e_, caudate lobe; _f_, gall-bladder; _g_, cystic duct;
_h_, hepatic ducts; _i_, common bile-duct; _j_, portal vein; _k_, part
of duodenum.]
The =Gall-bladder= (Fig. 101, _f_) is pear-shaped and lies in a cleft on
the caudal (or dorsal) surface of the right median lobe (_c_, _c′_) of
the liver. Its larger end is directed caudad (or ventrad) and is free.
By one surface it is in contact with the liver and not covered by
peritoneum, while the other surface is covered by peritoneum. The
peritoneum in passing from the larger free end to the liver forms one or
two ligament-like folds. By its smaller end the gall-bladder is
continuous with the =cystic duct= (_g_). This duct is about three
centimeters long and has a sinuous course. At its distal end it is
joined by two (or more) =hepatic ducts= (_h_), bringing the bile from
the lobes of the liver. The relation of these to the cystic duct varies.
They may open into it by a common trunk or separately. Of these hepatic
ducts one is made up by the junction of smaller hepatic ducts from the
left half of the liver and the left half of the cystic lobe, while the
other is similarly formed by smaller ducts from the right half of the
cystic lobe, from both divisions of the right lateral lobe and from the
caudate lobe. The duct formed by the junction of the hepatic and cystic
ducts is the =common bile-duct= (=ductus communis choledochus=) (_i_).
It passes in the free right border of the gastroduodenal omentum to the
duodenum (_k_) and opens into it by way of the ampulla of Vater, in
common with the pancreatic duct, at a point on the dorsal surface of the
duodenum and about three centimeters from the pylorus.
=Pancreas.=--The pancreas (Fig. 102, _a_) is a flattened, closely
lobulated gland of irregular outline, about twelve centimeters long,
varying in width from one to two centimeters. It is bent nearly at right
angles at about its middle. One of the halves (_a′_) into which it is
divided by its bend lies in the descending limb of the great omentum,
and is near the greater curvature of the stomach (_d_) and parallel to
it. The free end of this half is in contact with the spleen (_e_). The
other half (_a_) lies in the duodenal omentum between the limits of the
duodenal U (_c_) and reaches to the bottom of the U. The pancreas has
two ducts. The larger =pancreatic duct= (_b_) (sometimes known as the
duct of Wirsung) collects the pancreatic fluid from both halves of the
gland, the ductlets from each half uniting to make two larger ducts,
which then unite near the angle of the gland to make the pancreatic
duct. This is short and broad and opens into the ampulla of Vater
together with the common bile-duct. The =accessory pancreatic duct=
(duct of Santorini) opens into the duodenum about two centimeters
caudoventrad of the ampulla of Vater. It is formed by the union of
branches which anastomose with those of the pancreatic duct. It is
apparently sometimes lacking.
[Illustration: FIG. 102.--PANCREAS AND SPLEEN.
The œsophagus has been cut and the stomach turned caudad, so that the
dorsal surface of the stomach and the ventral surface of the duodenum
are seen. _a_, pancreas (_a_, duodenal portion; _a′_, gastric
portion); _b_, pancreatic duct; _c_, duodenum; _d_, stomach; _e_,
spleen.]
=Spleen. Lien.=--The spleen (Fig. 102, _e_) is a deep red, flattened,
elongated gland belonging to the lymphatic system. One of its ends, the
left, lies against the free end of the gastric half of the pancreas and
is broader than the other end. The spleen is curved and is suspended in
the descending limb of the great omentum so that it follows the greater
curvature of the stomach (_d_).