Digestive System Lectures

8/6/2019 Digestive System Lectures

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The Digestive System

Lecture 1

Bill Sellers

mailto:[email protected]

November 30, 1998

Contents

1 Introduction 1

2 Objectives 1

3 General Characteristics 2

4 The Mouth 3

4.1 Teeth . . . . . . . . . . . . . . . 4

4.2 Tongue . . . . . . . . . . . . . . 4

4.3 Swallowing . . . . . . . . . . . . 5

4.4 Salivary Glands . . . . . . . . . . 5

5 Oesophagus 5

6 Stomach 6

7 The Small Intestine 8

7.1 Duodenum . . . . . . . . . . . . . 8

7.2 Jejunum and Ileum . . . . . . . . 8

8 Pancreas 9

9 Liver 10

10 Gall Bladder 10

11 Large Intestine 11

11.1 C aecum . . . . . . . . . . . . . . 11

11.2 Colon . . . . . . . . . . . . . . . 11

11.3 R ectum . . . . . . . . . . . . . . 12

12 Anus 12

13 Portal Circulation 12

1 Introduction

The purpose of these lectures is to describe the

structures involved in digestion and to relate these

structures to their functions. The information I will

provide will follow very closely what is given in

Hole and Koos, chapter 12, but because my re-

search interest is in the interaction of form and

function over the last 100 million years or so, Iwill also try to provide an evolutionary context for

some of these observations.

2 Objectives

The objectives of these three lectures are as follows

(taken straight from the textbook):

1. Name and describe the location of the organs

of the digestive system and their major parts.

2. Describe the structure and general function of

each digestive organ and the liver.

3. Describe the structure of the wall of the ali-

mentary canal.

4. Explain how the contents of the alimentary

canal are mixed and moved.

5. Describe the mechanism of swallowing.

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6. Explain how the products of digestion are ab-

sorbed.

3 General Characteristics

The major components of the digestive system that

we will cover can be seen in figure 1. As you

can see, the digestive system in humans is basi-

cally a long tube and associated glandular struc-

tures. Food is ingested cranially and processed in-

crementally as it passes caudally. This processing

is both mechanical and chemical, leading to the ab-sorption of some of the chemical components of

the food items whilst other, indigestible compo-

nents are passed straight through. You will proba-

bly already be familiar with the basic structure and

the location of the major components, but we will

cover each organ in detail.

The muscular tube through which the food

passes is called the alimentary canal. The exact

histological appearance of this tube varies down

the length of the tube, but it basically consists of

four concentric layers (see figure 2). The inner-most layer is called the mucosa. This itself con-

sists of three thin layers: an epithelium lining the

inner surface, some underlying connective tissue

(lamina propria), and a small amount of smooth

muscle, confusingly named the muscularis mu-

cosae. The next layer is the submucosa. This

is loose connective tissue containing blood and

lymph vessels and nerves. The next layer is a

thicker muscular layer called the muscularis. In

most regions this consists of two layers of smooth

muscle: the inner one with fibres arranged circu-larly, and the outer one with fibres arranged longi-

tudinally. Rhythmic contraction of these muscles

pushes the contents of the tube steadily caudally.

This movement is called peristalsis. This mechan-

ical arrangement of fibres running perpendicularly

is very common in biological tubes whether it is

hydraulic skeletons in roundworms or or armadillo

penises. The precise angulation of the fibres de-

pends on the exact mechanical properties required.

Figure 1: Overview of digestive tract

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Figure 2: Idealized cross-section of gut tube

The outermost layer of the the gut tube is a thin

covering of visceral peritoneum - very similar in

structure to the pleura round the lungs. It is called

the serosa and consists of an outer epithelium with

connective tissue underneath.

Movement in the gut tube is not continuous. At

points, food is not moved, but kept in one place

and mixed. Obvious places where this occurs are

in the mouth and stomach. Peristaltic movements

are largely intrinsic to the smooth muscle of the

gut tube, however there is both sympathetic and

parasympathetic nerve supply. The parasympa-

thetic supply (rest and digest) tends to increase

peristaltic frequency and increase secretion of en-

zymes and mucus. The sympathetic supply tends

to decrease inhibit this and also contracts the arteri-oles, reducing the local blood supply. The sympa-

thetic supply also controls directly specific muscu-

lar structures called sphincters. These are regions

of thickened circular muscle that block the tube

when contracted, preventing the movement of gut

contents. The anal sphincter is slightly more com-

plicated, with a contribution from skeletal muscle

allowing a degree of conscious control of defeca-

tion.

Figure 3: The mouth

4 The Mouth

The mouth (see figure 3) is extremely complicated

anatomically because it has to cope with a variety

of different functions. Speaking, eating, breathing

are the obvious three, but certainly in humans and

other primates it is also an important manipulation

and tactile organ. It is also important in non-verbal

communication for display and sexual behaviour.

Humans and other so-called haplorhine primates

are unusual among mammals in that the sides of

the upper lip have fused which means that we donot have the wet, dog-like noses present amongst

most mammals.

The most important digestive role of the mouth

is mastication. This is the physical breakdown of

food by chewing. It is a complex process requiring

interactions between the highly muscular tongue,

cheeks and lips to position the food bolus between

the teeth, and strong crushing and shearing move-

ments of the tooth surfaces controlled by the mus-

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cles of mastication pulling on the jaws. The exact

configuration of jaws and teeth is highly variableamongst mammals, and is often used as a means

of classifying different mammalian groups. Hu-

mans, as omnivores, do not have specializations

for specific food items. This means that they have

very loose temporomandibular joints, and a great

deal of movement flexibility (depression, eleva-

tion, protrusion, retraction, circumduction, left and

right deviation). Mammals with more specific di-

ets have more specialized dentition and restricted

jaw movements. Carnivora, for example, can only

depress and elevate to any great extent and havescissor like teeth for shearing food. Individual

teeth also vary in shape and function.

4.1 Teeth

Humans have a dental formula of 2 1 2 32 1 2 3

. This

means that an adult human has 2 incisors, 1 ca-

nine, 2 premolars and 3 molars on their up-

per jaw, and the same on their lower jaw. Com-

monly, the 3rd molar may be missing, or may never

erupt. Incisors are single cusped, sharp edged cut-ting teeth used for biting chunks off a larger food

item. They are also used for removing the skins

of fruit. Human canines are extremely small com-

pared to most other primates, although they are

still classically pointed. In carnivores, canines are

used to immobilize prey, but in most primates they

are more important for communication (aggression

and as a secondary sexual characteristic). In hu-

mans, their role is largely as an accessory incisor.

The premolars are bicuspid, with flattened surfaces

and the molars generally have 4 cusps on the up-per jaw and 5 on the lower jaw although there is a

certain amount of variation here. These so called

cheek-teeth are where most of the chewing takes

place. A combination of occlusive, puncture-crush

forces and side-to-side shearing forces reduces the

size of the food particles so that the surface area

is increased to help digestion, and to produce a

soft, deformable, moist, lubricated bolus that can

be easily swallowed and moved by peristalsis. This

formation into a bolus may be more important than

the classic explanation of increasing surface area.It is likely that our digestive system can cope with

large particle sizes, but we all know how difficult

eating dry cream-crackers is.

4.2 Tongue

The tongue has several important functions in eat-

ing. Most obviously, the tongue contains the tastereceptors. These help identify the food value of

what has been put into the mouth. Things that taste

pleasant are likely to be nutrition, and things that

taste unpleasant are likely to be poisonous. Things

that are eaten, whether they are plants or animals,

are involved in an escalating evolutionary arms

race which their predators. In the case of plants,

many species produce various toxic and otherwise

unpalatable compounds to reduce their desirability

as food items. Interestingly, at least two species,

humans and common chimpanzees have realizedthat these compounds can have valuable medicinal

effects and can be considerably more toxic to var-

ious parasitic organisms than to the host, and they

actively seek out certain plants when they feel un-

well. In addition, many of the chemicals that give

certain spices a hot taste are actually plant toxins.

The other major functions of the tongue are me-

chanical. It moves the bolus of food between the

teeth whilst chewing, and in swallowing it moves

the food to the back of the buccal cavity to ini-

tiate swallowing. The tongue contains three setsof intrinsic muscles with fibres that run orthogo-

nally in the three anatomical directions (proximo-

distally, transversely and supero-inferiorly). These

allow the tongue to flatten or elongate. There are

also extrinsic muscles that attach the tongue to the

lower jaw and throat which allow it to move around

within the mouth (pro- and retraction, lateral devi-

ation, elevation and depression). It can also roll

within the mouth.

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Figure 4: Sagittal section of mouth and pharynx

4.3 Swallowing

Swallowing is a complex series of movements that

moves food from the buccal to the oesophagus (see

figure 4). It is made more complicated by the dual

role (respiration and digestion) of a number of the

structures in this region it is important that food

does not fall into the trachea and the lungs and also

important that it does not get pushed up into the na-

sopharynx. During swallowing the tongue pushes

the bolus backwards into the oropharynx. Other

muscles raise the soft palate and this action closes

off the nasopharynx. The back of the tongue andmuscles in the walls of the pharynx start to push

the bolus downwards by peristalisis and this action

pushes the epiglottis, a sprung, cartilagenous flap,

downward so that it covers the entrance to the tra-

chea whilst the food passes downwards to the oe-

sophagus. The epiglottis does not produce a per-

fect seal and choking occurs as a reflex reaction of

particles of food do accidentally fall into the tra-

chea.

4.4 Salivary Glands

Salivary glands in the mouth secrete saliva (spit)

(see figure 5). This fluid moistens food and con-

tains enzymes that start off starch digestion. It

also contains a variety of other chemicals with an-

tibacterial and anti-inflammatory functions. There

are many small salivary glands lining the tongue,

cheeks and palate. In addition there are three pairs

of large, discrete glands: the submandibular,

parotid and sublingual glands. Salivary glands

contain two sorts of secretory cells: serous cells

that secrete a watery fluid containing digestive en-

zymes; and mucous cells that secrete a viscous

fluid called mucus whose major role is to stick

food particles together to form the bolus. In sec-

tion, the serous cells stain strongly with H&E, but

the mucous cells remain largely unstained (figure

6). The parotid and submandibular glands form

a branching tree structure around their ducts, and

saliva is secreted through a single duct per gland.

The parotid duct enters the buccal cavity near the

upper second molar and the submandibular duct

opens underneath the tongue quite close to the

midline. The sublingual glands have many, sepa-

rate small ducts. Problems with saliva production

can lead to the formation of salivary stones which

can block the ducts and need to be removed surgi-

cally.

5 Oesophagus

The oesophagus is a straight, muscular tube con-

necting between the pharynx and the stomach. It

descends through the thorax behind the trachea andanterior to the thoracic aorta. The vagus nerves

exit the thoracic cavity through the diaphragm next

to the oesophagus. The wall of the oesophagus is

muscular (figure 7), and mucous glands in the mu-

cosa serve to keep the stratified, squamous epithe-

lia moist and lubricated. In the resting state the

oesophagus is deeply folded and it distends greatly

to allow the passage of food.

At the oesophageal hiatus (the hole in the di-

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Figure 5: Diagram showing the locations of the

major salivary glands

Figure 6: Diagram of a salivary gland

Figure 7: Cross-section of the oesophagus

aphragm), the muscular wall of the oesophagus is

thickened. These fibres are usually contracted to

prevent reflux of stomach contents. This action is

helped by a local thickening of muscle in the di-

aphragm which acts externally on the oesophagus

to help prevent both regurgitation and the physical

movement of the top of the stomach into the tho-

rax.

6 Stomach

The stomach is a large, classically J-shaped, bag-

like organ that hangs under the diaphragm on the

left-hand side (figure 8). It has a capacity of about

one litre, although, this can increase markedly. In

humans the stomach is used to mix food with gas-

tric juices, initiate the digestive process and store

food before passage to the small intestine. In many

mammals, notably ruminants, but also some leaf

eating primates, it is also a site for the fermentationof vegetable matter. This means that the stomach

contains a large number of micro-organisms that

are used to help break down otherwise indigestible

vegetable matter and neutralize toxins. This role

can be enhanced by having a multi-chambered

stomach that helps maintain optimal fermentation

conditions, and in some cases allows controlled re-

flux so that food can be chewed repeatedly.

The stomach can be divided into a number of re-

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Figure 8: The stomach

gions. The cardiac region is near the oesophageal

opening. The fundus is the area just above this

opening that is often seen containing air on abdom-

inal X-rays. The pylorus is the area near the exit of

the duodenum. This contains the powerful pyloric

sphincter which is thickening of the muscle wall

that prevents reflux from the small intestine back

into the stomach. The rest (and largest part) of the

stomach is the body.The stomach wall consists of a thickmucosa that

contains gastric pits (figures 9 and 10). These

are tubular gastric glands that in general contain

three types of secretory cells. Mucous cells secrete

mucus, chief cells secrete digestive chemicals and

parietal cells secrete a strong acid. This com-

bined product is the aforementioned gastric juice.

The role of the acid is to kill bacteria present in

the food. The digestive chemicals start breaking

down large food molecules into smaller compo-

nent molecules that can be absorbed, and the mu-cus helps prevent damage to the stomach wall by

lubricating against mechanical damage, and neu-

tralizing the effects of both the acid and digestive

chemicals.

The stomach is a very muscular organ. It has

three layers of muscle in the muscularis rather than

the normal two. In addition to the normal exterior

longitudinal muscle, and the deeper circular mus-

cle, it has an inner layer of oblique muscle. These

Figure 9: Diagram showing the variations in the

stomach wall

Figure 10: Cross-section of stomach wall

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muscles are used to churn the food to help the mix-

ing of food with gastric juice which produces amixture called chyme. A small amount of this is

squirted at intervals through the pylorus into the

small intestine.

7 The Small Intestine

The small intestine is a tubular long, tubular struc-

ture running from the exit of the stomach to the

beginning of the large intestine. It is mostly con-

cerned with continued food digestion and absorp-

tion of the products of digestion. It is divided into

a number of sections.

7.1 Duodenum

The first section of the small intestine is the duode-

num. This is a short (about 25cm long) C-shaped

tube that runs horizontally to the right from the

exit of the stomach, wraps itself around the head

of the pancreas, passing over the top of the right

kidney and then passes to the left before becoming

the next section of the small intestine (figure 11).

It is attached to the posterior abdominal wall rather

that being suspended on a fold ofperitoneum like

most of the rest of the abdominal alimentary canal.

This condition is known as retroperitoneal.

The duodenum is histologically distinct from the

rest of the small intestine because it contains extra

(Brunners) glands that secrete an alkaline mucus

that is used to neutralize the acidic chyme that is

produced in the stomach. It is also the region that

receives large ducts from the major digestive or-

gans: the liver and the pancreas.

7.2 Jejunum and Ileum

The next two sections of the small intestine are the

jejunum and the ileum. There is little real distinc-

tion between the two parts the jejunum is sim-

ply the proximal 2/5ths of this section (figure 12).

The jejunum and ileum are suspended by a fold of

peritoneum. The parietal peritoneum is attached

Figure 11: The small intestine

Figure 12: Diagram showing the variation between

different areas of the small intestine

to the abdominal wall and the gut tube acts rather

like a hose-pipe left underneath a carpet. At in-

tervals the length if this hose is such that it loops

away from the floor taking the covering carpet withit. So, the jejunum and ileum, several metres long,

hang in loops in the abdominal cavity, suspended

by a mesentery which consists of a double layer of

peritoneum which is continuous with the visceral

peritoneum surrounding the tube and with the vis-

ceral peritoneum attached to the posterior abdom-

inal wall. The blood and nerve supply to and the

lymph drainage from the loops of gut runs between

these two folds.

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Figure 13: Cross section of ileum

The role of the small intestine is digestion and

absorption (figure 13). The gut wall consists ofmillions of small, finger-like projections called

villi. Between the villi are intestinal glands which

contain mucus secreting goblet cells. The villi

contain a rich blood supply for transporting away

digestion products, and also blind-ended lacteals

which drain into the lymph vessels which also

carry away digestion products. The internal wall of

the small intestine is raised in circular folds called

plicae circulares. These help to increase the sur-

face area available for absorption. The villi further

increase the surface area and the cells in the colum-nar epithelium that line the villi have microvilli on

their lumenal surface again increases the surface

area available for absorption.

The small intestine is an extremely active tissue.

The constant movement of intestinal contents and

the energetic constraints of food absorption mean

that cells are quickly lost from the tips of the villi.

There is a constant migration of cells from the in-

testinal glands (also known as crypts) towards the

Figure 14: Diagram showing the relations of the

liver and pancreas

tips of the villi, so that within the crypts, there are

often signs of active cell division.

The small intestine ends at the ileocaecal valve

which is a sphincter that separates the contents of

the small intestine from the large intestine.

8 Pancreas

The pancreas, in the context of this series of lec-

tures, is an organ for the production and secretion

of digestive enzymes (figure 14) this is its ex-

ocrine function. It also has an important endocrinerole which will be dealt with elsewhere. It is a

long, thin organ that sits underneath the stomach

with its head region closely associated to the inte-

rior of the C of the duodenum. The pancreatic

duct runs the length of the body of the pancreas

and merges with the common bile duct before en-

tering the duodenum. The contents of the duct is

pancreatic juice which is a secretion produced by

acinar cells (figure 15).

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Figure 15: Section of pancreas

9 Liver

The liver is the largest gland in the body (fig-

ure 14). This is a somewhat misleading statement

since many of its functions are not glandular. How-

ever, it is an extremely large and important organ.

You dont live long with no liver! The liver takes

up the space on the right-hand side that is occupied

by the stomach on the left. It is firmly pushed upagainst the right dome of the diaphragm where the

inferior vena cava, which passed through the liver,

enters the thoracic cavity. It is held in place by

folds of peritoneum. This peritoneal folding pro-

duces the falciform ligament anteriorly which at-

taches to the ventral abdominal wall and the coro-

nary ligament superiorly which attaches to the di-

aphragm. The liver itself is divided into four lobes,

with the left and right lobe being by far the largest,

with the caudate and quadrate lobes being small

lobes near the vena cava and the gall bladder re-spectively.

Microscopically, the liver is organized into hep-

atic lobules. These consist of a small central

vein with columns of hepatic cells radiating out-

wards from this central point (figures 16 and 17).

Blood flows radially inwards towards the central

vein between the columns from both hepatic arter-

ies and branches of the hepatic portal vein. The

hepatic portal vein drains the portal venous sys-

Figure 16: Diagram showing the structure of the

liver

Figure 17: Section of liver

tem and contains high concentrations of digestion

products that are processed by cells in the liver

(hepatocytes). Also present in the lobules are hep-

atic macrophages that engulf most of the bacterial

cells that have entered through the intestinal wall.

Also within the lobules are bile canals. These are

fine tubes that drain the secretory product bile out

to the periphery of the lobule where the tubes unite

to form hepatic ducts which eventually unite to

form the common hepatic duct that carries bile

out of the liver.

10 Gall Bladder

The gall bladder is a small sac-like container that

attaches by a short tube (the cystic duct) to the

common hepatic duct (figure 14). Once the com-

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Figure 18: The large intestine

mon hepatic duct has joined the cystic duct it is

renamed the common bile duct which continues

to join with the pancreatic duct just before it emp-

ties into the duodenum. The bile duct stores thebile produced by the liver, and it is able to contract

to force bile into the duodenum as required by di-

gestion.

11 Large Intestine

The large intestine is the last part of the alimentary

canal. The large in the name refers to its diam-

eter rather than its length since the small intestine

is longer in humans. The large intestine consists ofthree functionally distinct parts, all those these are

further subdivided for descriptive purposes. These

major divisions are the caecum, the colon and the

rectum (figure 18).

11.1 Caecum

The caecum is a sac-like structure at the beginning

of the large intestine, situated retroperitoneally in

the lower right quadrant of the abdomen. In hu-

mans it is rather insignificant, but in many othermammals it is the alternative site for fermentation

of food. When this is the case, it can be larger

than the stomach. In humans, a small, blind-ended

tube hangs off the caecum. This is the vermiform1

appendix which has no digestive role, although it

may have a role as part of the lymphatic system in

fighting off infection.

11.2 Colon

The colon is a long tube that functions as the main

area for water and electrolyte absorption. In many

cases, this is actually re-absorption since large

quantities of both are secreted into the gut higher

up in the alimentary canal to aid digestion. The

colon is the region where these substances are re-

covered. Descriptively, it is divided into four re-

gions.

1. The ascending colon is the first part of the

colon which ascends vertically from the cae-

cum in the lower right quadrant up towards

the liver in the upper right quadrant. It is

retroperitoneal.

2. The transverse colons hangs down from a

complex mesentery more or less horizontally

from the end of the ascending colon in the up-

per right quadrant to the descending colon in

the upper left quadrant.

3. The descending colon runs retroperitoneally,

vertically down from the upper left quadrant

to the lower left quadrant.

4. The sigmoid colon is a short length of colon

that is once again attached to a mesentery that

connects the end of the descending colon to

the rectum. It is mostly contained within the

pelvic cavity.

1Vermiform means worm-like.

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Histologically, the wall of the colon contains a

thick mucosa containing many tubular, mucus se-creting glands (figure 19). Absorption occurs pri-

marily in the proximal half of the colon. The mu-

cus is mainly lubricating and protective to allow

the easy movement of the gut contents distally.

11.3 Rectum

The final region of the large intestine is the rec-

tum. This is a short tube that lies next to the sacrum

within the pelvic cavity (figure 20).

12 Anus

The external opening of the rectum is through the

anus. This forms the last 2 to 4 cm of the alimen-

tary canal. The mucous membrane lining the anal

canal is folded into a series of 6 to 8 longitudi-

nal anal columns. Free movement of faecal mat-

ter through the anal canal is prevented by a series

of ring-like sphincters. The internal anal sphinc-

ter is a ring of smooth muscle which is under in-

voluntary control. The external anal sphincter is

skeletal muscle under voluntary control. retroperi-

toneal.

13 Portal Circulation

The blood supply of most of the digestive system

is entirely normal blood flows in from nearby

arteries and is drained into nearby veins. How-

ever, the venous drainage for the parts of the gut in-

volved in substantial amounts of absorption of di-gestion products have a different venous drainage

(figure 21). Normal venous drainage where the

blood is conveyed directly back to the right atrium

of the heart is referred to as systemic. Venous

drainage from the gut drains first to the liver, and

only after that does it pass to the heart. This

drainage from one organ system directly to another

is referred to as the portal system. The reason

for this separate drainage is that the composition

Figure 19: Cross-section of colon

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Figure 20: The rectum and anus

of blood is greatly altered by digestion products,

and these products need to be processed before the

blood can be allowed to mix back with the rest of

the blood supply. This processing occurs within

the liver, so the liver has two blood inputs: the

hepatic artery which supplies oxygenated arterial

blood that provides the liver with the oxygen thatit needs; and the hepatic portal vein which is de-

oxygenated, venous blood drained from the distal

oesophagus, the stomach, the duodenum, jejunum,

ileum, the caecum, appendix, colon and the prox-

imal part of the rectum. It also drains blood from

the spleen, the pancreas and the gall bladder. There

are some areas where the portal and systemic sys-

tem anastamose. These are generally not impor-

tant, but in certain situations (notably portal hy-

pertension) they can open up to allow an alterna-

tive route for blood draining the gut. These includethe junctions between the areas of the oesophagus

and the rectum that are drained by portal and sys-

temic veins, and umbilical vein remnants left over

from the fetal circulation.

Figure 21: Diagram of the hepatic portal system

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Digestive System Lectures

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