1. Structure of the Small intestine

2. Parts of the small intestine
The small intestine is split into 3 parts the duodenum, jejunum and the ileum.
There are microscopic finger-like projections called villi covering the small intestinal walls which increase surface area for absorption.
Its also contains Intestinal juice (succus entericus)

3. Structure of the small intestine.
Duodenum: a short section that receives secretions from the pancreas and liver via the pancreatic and common bile ducts.
Jejunum: considered to be roughly 40% of the small gut in man, but closer to 90% in animals.
Ileum: empties into the large intestine; considered to be about 60% of the intestine in man, but veterinary anatomists usually refer to it as being only the short terminal section of the small intestine.

4. The duodenum.

5. The duodenum continued…
The duodenum contains
Mucosal layer made up of villi projections
Sub mucosa containing connective tissue, glands and blood vessels folds under the mucosal layer.
Muscularis with circular and longitudinal muscle layers.
Serosa made up of viseral peritoneum and mesentery.
Note the folding of the intestinal muscoa into structures called plicae circulares which increase absorptive surface area.

6. Microscopic Duodenum Villi
The outer edge of the villus is made up of simple columnar epithelium with goblet cells
The lacteal, core of the villus, is lymphatic tissue that absorbs digested lipids in the form of chylomicrons

7. Submucosal Microscopic Detail
Brunner's glands are only found in the duodenum. They secrete mucus.
There is a very thin layer of smooth muscle cells that forms the muscularis mucosa.

8. Duodenum Muscularis Microscopic Detail
Together the circular and longitudinal muscle layers work in a coordinated motion to cause the peristaltic movement of the GI tract.
The myenteric plexus of Auerbach located in the muscular layer controls the gastrointestional motility

9. Microscopic View of the Jejunum

10. The jejunum continued..
The jejunum, the middle section of the small intestine, is different from the duodenum due to lack of Brunner's glands. It is different from the ileum because it has less goblet cells and generally lacks Peyer's patches.
The jejunum is slung from the posterior abdominal wall by the mesentery of the small intestines and, therefore, is extremely mobile.

11. Continued again..
The jejunum is about 2.5m (8ft) long and passes imperceptibly into the ileum, which is about 4m (12ft) long. this part of the small intestine occupies a central position in the abdominal cavity, below the liver and the stomach, and behind the transverse mesocolon, the transverse colon and the greater omentum.

12. Villi of the Jejunum Mucosa
At the base of the many villi of the jejunum are simple tubular invaginations or pits that extend to the muscularis mucosae.

13. Submucosa of the Jejunum
Note the brightly stained collagen of the connective tissue

14. Muscularis Externa of the Jejunum
Consists of smooth muscle

15. Ileum Microscopic Cross Section

16. Ileum continued..
The ileum is different from the duodenum and the jejunum by having more goblet cells in the mucosa and lymph nodules called Peyer's patches.
Is also slung from the posterior abdominal wall by the mesentery of the small intestines and, therefore, is extremely mobile

17. Definitions
Mesentery-Those arteries and veins that supply the small and large intestines.
Lacteal-Pertaining to, or containing, chyle; as, the lacteal vessels