Solid line indicates mucous membrane.
]
The =submucous coat= is a layer of areolar tissue in which the vessels
and nerves ramify. It contains also the duodenal glands and the bases of
the solitary glands and Peyer’s patches.
The =mucous membrane= is soft and velvety. It has a grayish or
yellowish-red color and is very vascular. About five or six inches from
the pylorus it forms a pouch, the =diverticulum duodeni=, in which the
pancreatic and hepatic ducts open. On a small papilla opposite this is
the termination of the accessory pancreatic duct. At the ileo-cæcal
opening the mucous membrane projects slightly into the cavity of the
cæcum, forming the =ileo-cæcal valve=. The free surface is thickly beset
with villi, small projections of the mucous membrane which can be seen
well by placing a piece of the membrane in water. They are relatively
short and thick and have a distinct neck in the horse. Each contains a
central lymph vessel (lacteal), and around this a plexus of capillaries,
lymphoid tissue, and unstriped muscle-fibers. They are important agents
in absorption from the contents of the intestine. The epithelium is
columnar, with many goblet cells. Underneath the basement membrane is a
layer of unstriped muscle-fibers, the =muscularis mucosæ=.
The glands of the small intestine are of three kinds:
1. The =intestinal glands= (Glandulæ intestinales Lieberkuehni) are
found throughout. They are simple tubular glands which open between the
villi.
2. The =duodenal glands= (Glandulæ duodenales Brunneri) are found in the
first twenty feet (ca. 6 meters) of the bowel. They are racemose glands,
and are situated in the submucosa, so that their ducts perforate the
muscularis mucosæ and the mucous membrane.
3. The =lymph follicles= (Noduli lymphatici) are found either scattered
or in groups. In the former case they are termed =solitary glands=
(Noduli lymphatici solitarii), in the latter =Peyer’s patches= (Noduli
lymphatici aggregati). The solitary glands are about the size of a
millet-seed or a small sago grain. Peyer’s patches are situated chiefly
along the surface opposite to the mesenteric attachment and begin about
three or four feet from the pylorus. They number one to two hundred, and
are usually one to two inches (2 to 5 cm.) long and a quarter of an inch
to one-half inch (ca. 2 to 14 mm.) wide. Larger ones occur in the
terminal part, where one patch may have a length of seven to fifteen
inches (ca. 17 to 38 cm.) and a width of half an inch to one inch (ca. 5
to 25 mm.) in young horses (Ellenberger). They undergo atrophy in old
subjects.
[Illustration:
FIG. 265.—PEYER’S PATCHES OF SMALL INTESTINE OF HORSE.
]
=Vessels and Nerves.=—The =arteries= of the small intestine come from
the cœliac and anterior mesenteric arteries. The =veins= go to the
portal vein. The =lymph vessels= are numerous and go to the mesenteric
lymph glands. The =nerves= are derived from the vagus and sympathetic
through the solar plexus.