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The lungs are the spongy, pyramidal-shaped organs that house the bronchial

tree and the extensive pulmonary vascular network. Each lung is surrounded

by a thin mesothelial covering, the visceral pleura, and sits on either side of

Lungs

the heart within the thoracic cavity. The vascular and respiratory passageways enter each lung on

its medial aspect at the hilum. The wide base of the lung sits on the diaphragm inferiorly and

tapers to a narrow apex superiorly. The right lung has three lobes and the left lung two lobes.

Right lung

Lateral view, anterior to the right

Right lung

Medial view, anterior to the left

Photomicrograph of lung tissue

100x

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 1 Apex

 2 Superior lobe

 3 Middle lobe

 4 Inferior lobe

 5 Oblique fissure

 6 Transverse fissure

 7 Segmental (tertiary) bronchus

 8 Pulmonary artery

 9 Pulmonary vein

10 Costal impression

 11 Diaphragmatic surface

12 Aortic impression

13 Cardiac notch

Left lung

Lateral view, anterior to the left

Left lung

Medial view, anterior to the right

Photomicrograph of lung tissue

100x

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16 Small artery

17 Alveolar spaces

18 Blood vessels with rbcs

19 Lingula

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Cast of Trachea and Bronchial Tree

Latex cast of respiratory passageways of trachea and lungs of a dog

Anterior view at left, posterior view at right

 1 Trachea

 2 Right main (primary) bronchus

 3 Left main (primary) bronchus

 4 Lobar (secondary) bronchus

 5 Segmental (tertiary) bronchus

 6 Branching bronchiole network

The cast below is from a large dog’s lungs and is approximately the same size as human lungs. The casts were created by

forcing liquid latex into the respiratory passageways of the lungs and then letting the latex harden. The lungs were then

placed in a weak acid until the organic tissue of the lungs was digested away. These views of the cast allow you to visualize

the extensive nature of the bronchial tree as it branches out to the larger alveolar passageways within the lungs. The

smaller alveolar spaces did not get incorported into the casts.

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REAL ANATOMY

18 Digestive System

Th e digestive system is the extensive environmental interface that makes

it possible to transfer nutrients, water, and electrolytes

from the food we eat into the body’s internal environment. Th is is made possible by a complex lining,

which through a series of folds and a variety of small

to microscopic projections greatly increases the surface interface between the digested contents within

the gastrointestinal organs and the numerous small

capillaries beneath this lining. To better appreciate

the degree of this surface increase, realize that the

average total surface area of the skin of an adult

 human is about 20 square feet, while the surface

area of the digestive system is approximately 2,500

square feet, or about the size of a tennis court. To

make the transfer across this extensive surface area

possible, the food we eat must be broken down

into small molecules that can be absorbed from the

digestive tract into the circulatory system, which

then distributes the molecular metabolites to the

cells. Th erefore, the digestive organs also function in

the mechanical and chemical breakdown of the food.

Developmentally the digestive system begins as a

simple tube called the gut tube or gut. As this simple

tube develops into the highly convoluted organs of the

adult anatomy, it undergoes structural changes that account for its various functions. Th ough these structural

changes lead to diff erences in the tube from one region to

the next, there is a basic pattern of design throughout the

length of the tube. Th is structural pattern is responsible for

the general function of the digestive system. Modifi cations of

this pattern allow for the variation in structure and function

along its length. Th is chapter will highlight the structural variation and underlying design of the digestive system.

Find more information

about the digestive

system in

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Superfi cial dissection of abdominal viscera

Anterior view

Intermediate dissection of abdominal viscera

Anterior view

The digestive system begins at the mouth, where

food and drink enter this tubular organ system to be

processed by the teeth and tongue. From the mouth

Digestive System Organs

the broken-down food moves through the transport tube called the esophagus to the storage and mixing organ called the

stomach. The stomach thoroughly mixes digestive juices and mucous with the food as it tosses it around to produce a softened substance called chyme. The chyme is slowly moved into the small intestine where powerful digestive chemicals are

added from the pancreas. As the chyme slowly moves through the long small intestine, the digestive enzymes break it into

small metabolic fuel molecules that the intestine absorbs. The material that cannot be digested and absorbed is passed into

the large intestine where the nondigested remains are held until they can be removed through the anus as feces. The photos on this and the facing page depict the digestive organs and their related mesenteries.

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Intermediate dissection of abdominal viscera

Anterior view

Deep dissection of abdominal viscera

Anterior view

 1 Esophagus

 2 Stomach

 3 Liver

 4 Gallbladder

 5 Pancreas

 6 Duodenum

 7 Jejunum

 8 Ileum

 9 Cecum

10 Ascending colon

 11 Transverse colon

12 Descending colon

13 Rectum

14 Greater omentum

15 Lesser omentum

16 Mesentery

17 Transverse mesocolon

18 Lungs

19 Heart

20 Diaphragm

21 Aorta

22 Spleen

23 Trachea

24 Inferior vena cava

25 Kidney

26 Bladder

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Photomicrograph of small intestine wall

40x

The wall of the digestive tract has a basic pattern of

design  that is found throughout its length. This pattern

consists of three tunics or layers of anatomy. The tunica

Design of the Gut Wall

mucosa and its subdivisions, including the tela submucosa, form the inner layer of the wall and consist of an extensive

epithelial lining with an underlying vascullar connective tissue. The middle layer, or tunica muscularis, consists of smooth

muscle that provides for the varied types of movements that occur within the digestive organs. The majority of the organs

have an outer layer, the tunica serosa, comprised of a lubricated meosthelial membrane that reduces friction as the organs

move against one another. The image below, from the small intestine, illustrates the basic layers of the digestive tract wall.

Tunica mucosa

consisting of:

epithelium,

lamina propria,

and

muscularis

mucosae

Tunica serosa

Tela

submucosa

Tunica muscularis

circular layer

Tunica muscularis

longitudinal layer

 1 Simple columnar epithelium

 2 Lamina propria

 3 Muscularis mucosae

 4 Submucosal (Brunner’s) glands

 5 Villi

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 1 Lips

 2 Teeth

 3 Tongue

 4 Hard palate

 5 Soft palate

 6 Nasopharynx

 7 Oropharynx

 8 Laryngopharynx

 9 Parotid gland

10 Submandibular gland

 11 Parotid duct

12 Serous acini

13 Mucous acini

14 Vein

15 Trabecula

16 Masseter

17 Sternocleidomastoid

18 Sphenoid sinus

19 Epiglottis

20 Vertebral column

21 Cerebrum

22 Spinal cord

Dissection of head showing salivary glands

Lateral view

Sagittal section of head and neck

Medial view

The mouth, or oral cavity, is the entryway into the digestive system.

In addition to serving as the portal to the tubular gut, the mouth

contains structures, such as the tongue, teeth, and salivary glands,

Mouth and Pharynx

that help initiate the digestive process. The boundaries of this region are defi ned by the lips and cheeks, which form the

anterior and lateral walls, the palate, which forms the roof, and numerous muscles, the most conspicuous being the muscles

of the tongue, which form the fl oor of the mouth. The pharynx is the fi rst portion of the gut tube and is divided into three

regions. Each region communicates with a different cavity — the nasopharynx with the nasal cavity, the oropharynx with the

oral cavity, and the laryngopharynx with the cavity of the larynx.

Photomicrograph of submandibular gland

240x

Photomicrograph of parotid gland

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salivary glands

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Pharynx, esophagus, and stomach

Anterior view Photomicrograph of esophageal wall

40x

Step dissection of esophagus

Anterior view

Below the laryngopharynx the gut tube branches into an anterior respiratory tube, the larynx and a posterior digestive tube, the esophagus.

The esophagus is a narrow, collapsed muscular tube coursing from the

Esophagus

laryngopharynx to the stomach. It is approximately 25 cm in length and begins near the level of the sixth

cervical vertebra, where it runs inferiorly against the anterior surface of the thoracic vertebral column.

At the level of the tenth thoracic vertebra it deviates slightly to the left passing through the esophageal

hiatus of the diaphragm to enter the stomach. It functions as a muscular tube of transmission.

 1 Esophagus

 2 Tunica mucosa

 3 Tela submucosa

 4 Tunica muscularis circular layer

 5 Tunica muscularis longitudinal layer

 6 Stratified squamous epithelium

 7 Lamina propria

 8 Muscularis mucosae

 9 Tunica adventitia

10 Stomach

 11 Pharynx - dorsal wall

12 Vagus nerve

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Frontal section of stomach

Anterior view Photomicrograph of stomach mucosa

with callout above

40x and 100x

 1 Stomach

 2 Cardia of stomach

 3 Fundus of stomach

 4 Body of stomach

 5 Pyloric antrum

 6 Pyloric canal

The stomach is a J-shaped organ of variable size and shape and has the greatest diameter of any part of the gut tube. It occupies the upper left quadrant of

the abdominal cavity, where it is anchored to the posterior abdominal wall by a

Stomach

 7 Pylorus

 8 Pyloric sphincter

 9 Gastric rugae

10 Greater curvature

 11 Lesser curvature

12 Gastric pit

mesentery. The stomach performs several functions, the most important of which is to store ingested food

until it can be emptied into the small intestine at a rate that allows for optimal digestion and absorption.

13 Surface mucous cell

14 Lamina propria

15 Mucous neck cell

16 Gastric glands

17 Liver

18 Gallbladder

19 Spleen

20 Greater omentum

Abdominal dissection revealing stomach

Anterior view

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Small intestine in situ

Anterior view

The small intestine is a highly coiled tube with a fairly consistent diameter from beginning to end. It is approximately

6 to 7 meters long in the cadaver but, because of its muscle

Small Intestine

tone only around 4 to 5 meters in the living. The small intestine occupies the greater part of the mid- to

lower abdominal cavity and consists of three regions. The retroperitoneal fi rst part is called the duodenum and is about 30 cm in length. This C-shaped region receives the secretions from the pancreas and

liver. The remaining parts of the small intestine are the jejunum and ileum, which make up the bulk of

the organ and are attached to the posterior wall of the abdomen by the mesentery. The small intestine

is the principal site of digestion and absorption.

Sectioned small intestine

revealing circular folds

Internal view

Entire small intestine sectioned to show changes in

internal surface from the duodenal end to the ileal end

Internal view

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Loop of small intestine from unembalmed cadaver, opened to show circular folds

Anterior view

Loop of small intestine

Anterior view

 1 Duodenal end

 2 Ileal end

 3 Jejunum

 4 Ileum

 5 Circular folds

 6 Simple columnar epithelium

 7 Goblet cell

 8 Lamina propria

 9 Liver

10 Stomach

 11 Cecum

12 Transverse colon

13 Descending colon

14 Mesentery

15 Microvillus brush border

Photomicrograph of cross-section of intestinal villus

400x

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 1 Tail of pancreas

 2 Body of pancreas

 3 Head of pancreas

 4 Uncinate process of pancreas

 5 Pancreatic duct (of Wirsung)

 6 Major duodenal papilla

 7 Exocrine acinus

 8 Pancreatic ductule

 9 Pancreatic islet (endocrine cells)

10 Trabecula

 11 Duodenum

12 Liver

13 Gallbladder

14 Common bile duct

15 Spleen

16 Diaphragm

The pancreas is a pinkish glandular structure situated posterior to the stomach

in the retroperitoneal space of the abdominal cavity. It arises as an outgrowth

of the duodenum during development and retains this connection via the panPancreas

creatic duct. It is a dual glandular organ consisting of both exocrine and endocrine glandular tissue. It has

four basic regions: a head, neck, body, and tail. The exocrine glands and ducts produce and deliver the

powerful digestive enzymes to the small intestine.

Photomicrograph of pancreas

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Abdominal dissection with part of liver

and peritoneal organs removed

Anterior view

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