Hi :) This post will take a look at the anatomy of the lower alimentary tract including the small intestine, large intestine. We'll also discuss the blood supply, innervation and lymphatic drainage to this area of the alimentary tract.
Small Intestine
The small intestine extends from the pyloric orifice to the
iloecolic orifice. Proximally the small intestine consists of the duodenum
which is attached to the liver. Distally, the duodenum becomes more mobile and
is attached to the jejunum and ilieum.
The duodenum lies opposite the 9th intercostal
space and is about 25cm long. A cranial portion runs towards the liver where it
forms the cranial duodenal flexure. After this point, it becomes the descending
duodenum that runs caudally to below the tuber coxae. Here, after a medially
directed flexure, it runs cranially as the ascending duodenum to join the
jejunum at the root of the mesentery.
Most of the small intestine is the highly coiled jejunum.
Caudally the small intestine ends in a short portion called the ileum that
leads to the large intestine via the ileocolic valve. Near this area is the
caecocolic valve, which joins the large intestine with the caecum.
Horse
The horse has a simple small intestine but a highly modified
large intestine.
The descending duodenum lies dorsally on the right hand
side. It then runs caudally to the base of the caecum where it turns cranially
and continues as the ascending duodenum. The ascending duodenum runs around the
base of the caecum and then travels cranially to below the left kidney and ends
as the jejunum. The duodenum is supported by a short mesoduodenum.
The jejunum is highly coiled and lies dorsally on the left
hand side. The last meter of the small intestine is the ileum.
The large intestine begins in the dorsal part of the
abdominal cavity as the ascending colon. The ascending colon ends at the right
(hepatic) colic flexure where it becomes the transverse colon. This section
ends at the left (splenic) colic flexure and continues as the descending colon
that runs caudally and becomes the rectum which terminates at the anal orifice.
The colon is supported by mesentery called the mesocolon. Loops of jejunum lie
ventral to the colon.
Ruminants
The ascending colon is highly modified in ruminants to
increase surface area. It forms an S bend and then leads into a spiral coil.
The coil leads into another S bend and then the transverse and descending colon
which ends in the rectum. The ascending colon is suspended by the mesocolon.
The spiral portion of the colon is composed of centripetal loops which spiral
towards the centre where there is a central flexure and then form centrifugal
loops which spiral outwards.
Pigs
The small intestine of the pig is the same as in the dog.
The caecum is large, wide and blind-ending and is continuous with the colon
cranially. It lies on the ventral abdominal wall and has three taeniae. The
caecum lies to the left. The ascending colon also has spiral loops but are
different to that seen in ruminants as it does not have proximal or distal
loops. The spiral colon is helical and has three centripetal and centrifugal
coils which lie adjacent to each other.
The final centrifugal loop runs cranially on the right of
the stomach and crosses to the left hand side of the abdomen were it becomes
the transverse colon. It then runs caudally as the descending colon and ends at
the rectum in the pelvic cavity.
Taenia support the intestines to prevent tearing and are
also the sites of attachment of the mesocolon. The caecum has three, the
centripetal coils have two while the centrifugal coils have none.
Horse
The large intestine in the horse is about 8 meters long, has
a wide lumen as well as taenia and is sacculated.
The caecum is a
large, sacculated blind diverticulum and lies on the right hand side. The
parietal surface of the caecum is in contact with the right abdominal wall, the
diaphragm, duodenum and liver. Its visceral surface is in contact with the left
and right colon, mesentery and small intestine. The right ventral colon is
attached to the body of the caecum by the caecocolic fold. The base of the
caecum is attached to the right kidney and pancreas. The caecum has 4 taenia.
The ileocaecal valve opens on the lesser curvature of the
base of the caecum and is enclosed by a sphincter.
The caecocolic orifice lies about 5cm from the ileal opening
and the two are separated by a large fold.
The colon has
ascending, transverse and descending segments. The ascending colon is made up
of two parallel parts which are joined by connective tissue and fat.
- The Right Ventral Colon begins at the caecocolic orifice and passes cranioventrally and forms the sternal flexure.
- The Left Ventral Colon continues after the sternal flexure and runs caudally to the pelvic inlet. Here it bends dorsally and forwards to form the pelvic flexure.
- The left dorsal colon follows this and runs craniodorsally to the diaphragm. Here it turns at the diaphragmatic flexure.
- The Right Dorsal Colon follow the diaphragmatic flexure and runs caudally to the medial surface of the base of the caecum.
From here it runs dorsally to become the transverse colon
and passes from right to left immediately cranial to the root of the mesentery.
The descending colon begins ventral to the left kidney and
caudal to the saccus caecus. It is highly convoluted among the coils of the
small intestine and terminates at the rectum.
Taenia:
- Caecum and ventral colon: 4
- Pelvic flexure: 1
- Dorsal colon: 3
- Small colon: 2
- Small intestine: 0
The cranial and caudal mesenteric arteries supply most of
the intestines. The hepatic branch of the coeliac artery supplies the proximal
part of the duodenum.
The gastric, jejunal, and colic veins drain the intestines
and empty into the portal veins.
The intestines receive both parasympathetic and sympathetic
nerve supply from the submucosal and the myenteric plexuses.
Lymph nodes exist within the mesenteries and an extensive
network of vessels drain into the cisterna chyli and then to the thoracic duct.
That's it for this post, let me know if you have any questions :)
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