HUMAN ANATOMY – VOLUME 1

Fig 151. Construction of peritoneum.

1–5 – layers of peritoneum (without vessels); 6 — deep cribrosal collagen-elastic layer (with blood vessels); 7 — smooth muscles; 8 — serosal-muscular network of blood vessels.

The right mesenteric sinus contains some loops of the ileum. The l e f t m e s e n t e r i c s i n u s is limited by the descending colon on the left and the root of the small intestine mesentery on the right. At the bottom it communicates with the pelvic cavity. Retroperitoneally, within the boundaries of this sinus, lies the ascending part of the duodenum, lower half of the left kidney, end part of the abdominal aorta, left ureter, vessels, nerves and lymph nodes. This sinus primarily contains loops of the jejunum.

The parietal peritoneum, which forms the posterior wall of the peritoneal cavity, forms recesses (fossae), which may become sites of retroperitoneal herniae. The s u p e r i o r and i n f e r i o r d u o d e n o j e j u n a l r e c e s s e s are situated above and below the duodenojejunal flexure. The s u p e r i o r and i n f e r i o r i l e o c a e c a l r e c e s s e s are situated above and below the ileocecal valve. Beneath the cupola of the cecum is the r e t r o c e c a l r e c e s s. To the left of the sigmoid mesocolon lies the sigmoid recess.

In the cavity of the lesser pelvis the peritoneum forms recesses between its organs. In men the peritoneum covers the anterior surface of the rectum, passes onto the posterior and superior surfaces of the urinary bladder, and then continues into parietal peritoneum of the anterior abdominal wall. Between the bladder and rectum there is a r e c t o v e s i c a l p o u c h. At the sides it is bordered by the r e c t o v e s i c a l f o l d s, which stretch

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between the rectum and the bladder. In women the peritoneum passes from the rectum onto the back of the vagina, passes over the uterus and onto the urinary bladder in the front (Fig. 150). Between the uterus and the bladder is the v e s i c o u t e r a l p o u c h. Between the uterus and rectum is the deep r e c t o u t e r i n (Douglas’s) p o u c h. At the sides this recess if limited by the r e c t o u t e r a l f o l d s.

The peritoneum consists of mesothelium, collagen and elastic networks, as well as smooth muscle cells (Fig. 151).

Questions for revision and examination

1.Name the retroperitoneal, mesoperitoneal and intraperitoneal organs of the abdominal cavity.

2.Name the folds and fossae on the posterior surface of the anterior abdominal wall, below the umbilicus.

3.Name the walls of the omental bursa and the location of its recesses. Name the walls of the hepatic and pregastric bursae.

4.What structures border the paracolic grooves and mesenteric sinuses?

5.What recesses (fossae) are found on the posterior abdominal wall and are possible sites of hernia formation?

DEVELOPMENT OF THE DIGESTIVE SYSTEM

Beginning on day 20 of the intrauterine development the intestinal ectoderm of the embryo rolls into a tube, forming the primitive gut. The primitive gut is situated in front of the chorda, and is closed in the front and the back. The ectoderm forms into the epithelium and glands of the digestive tract (excluding the mouth and anal regions). The other layers of the digestive tube develop from the splanchnopleura, which is the medial unsegmented lamina of the mesoderm, which adjoins the primitive gut.

During week 3 of embryogenesis ectodermal depressions appear in the cranial (stomodeum) and caudal (proctodeum) ends of the embryo. The stomodeum protrudes into the cranial end of the primitive gut. The membrane between the stomodeum and the gut (oral membrane) erupts during the fourth week. The cloacal membrane, which separates the proctodeum from the cavity of the primitive gut, erupts somewhat later.

During the fourth week the ventral wall of the primitive gut protrudes forward, later to form the trachea, bronchi and lungs). This protrusion marks the border between the caudal (pharyngeal) and the truncal guts. The truncal gut is divided into an anterior, middle and posterior guts. The ectodermal lining of the stomodeum develops into the epithelium of the oral cavity. The caudal gut develops into epithelium of the pharynx. The anterior gut forms the epithelium of the oesophagus and stomach; the middle gut — the cecum, ascending and transverse colons and epithelium of

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the liver and pancreas. The posterior gut develops into epithelium of the descending and sigmoid colons and rectum. The peritoneum is formed by the somatopleura and visceropleura.

Development of walls of the oral cavity, bones of the facial skull and some internal organs takes place through development of the branchial apparatus of the embryo. On each lateral wall of the pharyngeal gut there are five protrusions (branchial recesses), between which there are thickenings called the branchial arches. The first (maxillary) and second (hyoid) arches are called visceral, and the lower three — branchial arches. The fist visceral arch develops into the superior and inferior walls of the oral cavity, the maxilla and mandible, lips, small bones of the middle ear (malleus and incus) and muscles of mastication. Tissue of the second visceral arch develops into the lesser cornua and body of hyoid bone, styloid process of temporal bone, stapes bone of the middle ear and muscles of facial expression. The first branchial arch develops into the greater cornua of hyoid bone. The other branchial arches form the cartilages of the larynx. Epithelium of the first branchial recess forms the epithelial lining of the tympanic cavity, auditory tube, etc. The second branchial recess develops into epithelium of the tonsillar fossa; the third and fourth recesses — epithelial components of the thymus and parathyroid glands.

The tongue begins to form during week 5 of embryogenesis from one unpaired ectodermal germ, which forms its tip and middle part, and a pair of germs, which form the posterior part and its root. These germs gradually merge with each other. The papillae of the tongue form during months 6 and 7.

The teeth form from the ectoderm, which covers the maxillary and mandibular alveolar processes. An ectodermal dental plate (thickening) gradually lowers into the mesenchyme of these processes. The pulp of the teeth has a mesenchymal origin.

During the second month of embryogenesis the primitive gut undergoes complex changes. A primitive intestinal loop appears, its flexure directed towards the umbilical opening. The gut protrudes through this opening, forming a physiological umbilical hernia. During the fourth month the umbilical opening narrows and the intestinal loops are drawn back into the body cavity.

During the second month of embryogenesis the future stomach begins to form as a dilation of the anterior gut. Beneath the primitive intestinal loop appears a small protrusion of the gut, which later develops into the cecum. The descending part of the primitive loop develops into the small intestine, and the ascending part—into the large intestine. The beginning of the descending part develops into the duodenum, while the rest of it becomes mesenteric small intestine. All parts of the intestine continue

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to grow and shift their position inside the abdominal cavity of the fetus (see below). Dorsally to the germ of the cecum form the future left colic flexure and transverse and descending colons. By month 6 of embryogenesis the ascending colon and right colic flexure appear. The end part of the large intestine develops into the sigmoid colon. The rectum emerges from the large intestine by formation of transverse septa in the wall of the cloaca. This septa divides the cloaca into a urogenital (anterior) and perineal (posterior) parts. After the breaking of the cloacal membrane the rectum becomes communicated with the outer environment.

Simultaneously with differentiation and growth, the different parts of the gut change their position within the future abdomen. During months 2 and 3 the posterior gut shifts out of the median plane upwards and to the left, in front of the intestinal loop. The intestinal loop turns 180 degrees clockwise. The germ of the cecum shifts upward and to the right, while the upper part of the intestinal loop shifts downward, behind the cecum. As the intestinal loop grows, the cecum germ becomes shifted downward, into the right iliac fossa. The intestinal loop forms a 90 degree flexure to the right. Its descending part becomes longer and forms numerous loops, which considerably shift the future large intestine. As a result, the ascending colon lies in the right part of the abdominal cavity, and the transverse colon lies across.

The peritoneal lining of the intestine develops together with formation of the mesentery of the primitive gut. During the first month of embryogenesis the truncal gut (below the diaphragm) is attached to the anterior and posterior walls of the embryo by the ventral and dorsal mesenteries, which originate from the splanchnopleura. The ventral mesentery below the umbilical foramen disappears at an early stage, while its upper part develops into the lesser omentum and the falciform ligament of the liver. The position of the dorsal mesentery is changed by the growing greater curvature of the stomach and its shift downward and to the right. As the stomach grows downward and shifts from a sagittal position into a transverse one, the dorsal mesentery protrudes from underneath its greater curvature, forming the greater omentum. The posterior part of the dorsal mesentery continues onto the posterior abdominal wall. The dorsal mesentery develops into mesenteries of the small and large intestines.

The anterior wall of the developing duodenum forms a pair of protrusions into the ventral mesentery, which later become the liver and gallbladder. The pancreas forms out of the merged ventral and dorsal endoderm protrusions into the dorsal mesentery. As a result of turning of the stomach and growth of the liver the duodenum and pancreas move into a retroperitoneal position and loose their mobility.

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AGE CHARACTERISTICS OF THE DIGESTIVE SYSTEM ORGANS

Oral cavity. In newborns and in children several months old the oral cavity is very small, the vestibule is narrow and the hard palate is wide and flattened. The alveolar margins (processes) of the maxilla and mandible have two grooves. The medial (internal) groove corresponds to germs of the deciduous teeth, and the lateral (external) groove—to germs of the permanent teeth. The soft palate is sufficiently developed. The cheeks are prominent due to the presence of the adipose body, which later move to the back and gradually disappears. In the middle of the upper lip of newborns there is a tubercle, which is connected with the gingiva by a short frenulum. In the middle of the inferior lip there is an impression, which disappears with age. In newborns the teeth are absent. The mucosa of the oral cavity is relatively thick, although in the regions of the lips and cheeks it is thin and movable. The mucosa of the hard palate forms transverse palatine folds. A newborn has less small salivary glands than an adult.

Major salivary glands. The average mass of the sublingual gland of a newborn is 0.42 g; of the submandibular gland — 0.84 g; of the parotid gland — 1.8 g. These glands develop most intensely between the fourth month and second year of life, when they increase in size and mass, and their secretory compartments undergo final differentiation. These glands reach maximal development between ages 20 and 29. After the age of 60 years the amount of parenchyma decreases, while the amount of stroma tends to increase. Their excretory ducts increase in size and often form ampulla shaped dilations.

Tongue. In newborns the tongue is broad and thick, its size is disproportionate relative to the oral cavity. The border between the body and root of the tongue resembles a deep groove. The lingual frenulum is better developed than in adults. The muscles of the tongue are formed and the papillae are well developed and contain numerous taste buds. The mucosa of the tongue contains less glands then in an adult. At 1.5 years of age the tongue is 4–5 cm long and 3.5 cm wide. The size of the tongue gradually increases with age. During senescence the papillae atrophy and the mucosa becomes thin.

Pharynx. The pharynx of a newborn is relatively wide (1.2–1.5 cm) and is 4 cm long. At this age it is situated higher (especially the laryngopharynx) then in an adult. The pharyngeal openings of auditory tubes resemble fissures, and are situated at the level of the hard palate. The auditory tube lies horizontally, which makes it easier for infections to spread into the tympanic cavity. The vault of the pharynx in newborns is flattened and leans forward relative to the oropharynx. The choanae are weakly

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developed. After birth, with intensive growth of the facial skull, the nasopharynx considerably increases in size. During puberty the pharynx primarily grows in length.

Oesophagus. In newborns the oesophagus resembles a thin tube 8– 10 cm long. Flexures and constrictions are not yet formed (except for the diaphragmatic one). They appear at ages 5–6. At ages 1–2 years the length of the oesophagus reaches 10–12 cm, and its diameter — 0.5–0.6 cm. Mucosal folds appear in the oesophagus at two years of age. With age its skeletotopic boundaries shift downward. At ages 20–29 the length of the esophagus is 23–30 cm, and thickness of its walls is 500–800 mm. During senescence its length slightly increases, and its walls become thinner. The number of glands tends to decrease during this period.

Stomach. In newborns the stomach is spindle-shaped. Its fundus is weakly developed, the body is relatively wide, the pyloric region is narrow, and the angle between the fundus and abdominal esophagus (angle of His) is acute. The surface area of the mucosa is 40–50 cm2, and the volume of the stomach is 30–35 cm3. The muscularis is weakly developed. The glands of the mucosa are differentiated and contain a lot of epitheliocytes, which produce lipase and lactase for digestion of milk. After birth the stomach grows quite quickly. Its volume and mass increases by several times after just two weeks. The size and shape of the stomach in an adult, as well as its position, thickness of its walls and number of gastric glands are highly variable. After age 60 the stomach often shifts downward (gastroptosis).

Small and large intestines. The length of the intestine in newborns is 340–460 cm. During the first year of life it increases by 50 percent. The ratio between the lengths of the intestine and that of the body is 8.3:1 in newborns; 5.4:1 at one year; 7.6:1 at age 16; and 5.4:1 in adults. The ratio between the large and small intestines is 1:5 in newborns and 1:4 in adults. In children the duodenum is situated higher than in adults, and often has the shape of a ring. In newborns and nurslings the duodenum is relatively longer than in adults. The bile duct and pancreatic ducts open into its superior part. The beginning of the mesenteric part of the small intestine is situated higher than in adults (up to the level of the L1 vertebra). The ileocaecal valve is also situated relatively high, below the liver. By age 14 it descends into the iliac fossa. In elderly people it may be situated inside the lesser pelvis (colonoptosis).

The caecum of newborns is 1.7 cm wide and 1.5 cm in length. During the first years of life the appendix is relatively longer than in adulthood (4–5 cm in newborns). The ascending colon in newborns is short, and the right colic flexure is shifted to the right by the relatively large liver. The

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transverse, descending and sigmoid colons are relatively long. The length of the transverse colon reaches 20 cm, and of its mesentery — 2 cm. The descending colon is approximately 5 cm long. The sigmoid colon (20 cm long) lies high in the abdominal cavity and has a long mesentery. A wide loop of this colon extends to and adjoins the rectum. By age 5 the sigmoid colon lowers into the inlet of the lesser pelvis. The rectum of a newborn is 5–6 cm long, and its ampulla and flexures are not expressed until ages 4–7. The most active growth of the rectum takes place after age 8.

In newborns the villi of the small intestine are short, and their number is less than in adults. Folds of the small and large intestine (except for anal columns) are weakly developed. The haustra, taeniae coli and epiploic appendages of the large intestine are not well expressed and develop during ages 4–5. The number of intestinal glands is less than in adults. The submucosa is very loose especially in the end parts of the large intestine. The maximal surface area of mucosa, length, number of glands and other parameters of the intestine are reached between ages 20 and 49. During senescence the taeniae coli become thin, the haustra and folds decrease in number and size, and the total number of intestinal glands becomes less.

Liver. In newborn children the liver is relatively large and movable. It occupies more than half of the abdominal cavity. It weights approximately 135 g, which is 4–4.4 percent of the body mass (in adults the liver makes up 2–3 percent of total weight). The longest transverse size of the liver in newborns is 11 cm; longest longitudinal size is 7 cm; longest vertical size is 8 cm. The left lobe of the liver is as large as the right one (sometimes larger). Its inferior margin protrudes 2.5–4.0 cm from underneath the costal arch, occasionally reaching the iliac crest. The relative size of the liver decreases with age, although its absolute size increases. At age 7 its inferior margin no longer extends below the costal arch, and its mass reaches 700 g. The liver reaches its final size between ages 20– 29. After 70 years its mass decreases, and its connective tissue expands. Hepatocytes contain an increased amount of lipofuscin. The number of dividing hepatocytes becomes less, and their nuclei become bigger.

The gallbladder in newborns is approximately 3–3.2 cm long and 1 cm wide. Its fundus does not extend below the inferior liver margin. At this age the gallbladder projects onto the anterior abdominal wall below the costal arch, 2 cm to the right of the anterior median line. By ages 10– 12 the volume of the gallbladder doubles. It reaches its final size by 20– 25 years. During senescence there is thinning of its walls, which may form diverticula (especially in the region of its neck).

The pancreas of a newborn is approximately 4–5 cm long and weights 2–3 g. It is situated higher than in an adult. In newborns it is less fixed to the

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abdominal wall, and is therefore more movable. It receives an abundant blood supply; its endocrine component is well expressed, while its exocrine part is still underdeveloped. The position of the pancreas relative to other organs, as is characteristic for adults, becomes established by age one. At age 3 the mass of the gland reaches 20 g, and by ages 10–12 it is 30 g.

Peritoneum. In newborns the peritoneum is thin and colorless. The submesothelial connective tissue layer is continuous. Later it gains a lacy appearance, resembling a network of connective tissue septa with blood vessels. The lesser omentum is well developed, and the epiploic foramen is relatively large. Recesses and folds of parietal peritoneum are well developed at this age, especially on the front abdominal wall. The greater omentum in newborns is short, and covers the small intestine loops only partially. With age the greater omentum becomes longer, thickens and accumulates a large amount of adipose tissue and lymphoid nodules. Often with age, especially in elderly people, adhesions may form between the parietal and visceral sheets of peritoneum, which may have an effect on the functioning of organs.

Questions for revision and examination

1.Describe the first stages of development of the digestive system (during first and second months of embryonic development).

2.Describe the changes, which take place after birth in structures of the oral cavity and its organs, pharynx and esophagus.

3.What age characteristics (changes) of the stomach, small and large intestine can you name?

4.Describe the age characteristics of the liver, gallbladder, pancreas and peritoneum.

VARIANTS AND ANOMALIES OF THE DIGESTIVE ORGANS

Lips. Total or partial cheiloschisis (cleft lip) may occur on the upper lip, usually lateral of the median groove (condition known as «harelip»). Sometimes the cleft extends to the wing of the nostril. Very rarely this fissure reaches the olfactory region of the nose or, avoiding the nostril wing, reaches the orbit and splits the lower lid. The lower lip may also be cleft. A very rare condition is an absence of one or both lips. The mouth fissure may be asymmetrically extended into one or both sides (macrostomia). It may also be narrowed (microstomia).

Palate. Cleft palate is a defect of the hard palate. It may be combined with cleaving of the soft palate. There are also various combinations of cleft palate and cleft lip. Sometime during this condition the alveolar process of the maxilla becomes separated from the rest of the bone by a deep fissure from one or both sides. Sometimes the uvula may be cleft. There

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are also cases when the uvula is displaced to one side or when its base begins on the posterior edge of the vomer. The shape and size of the uvula are variable. Muscle of the soft palate may very depending on the extent of non-union of the two halves of the palate. Sometimes there is a pterygotubal muscle, which originates from the medial pterygoid muscle. The pterygotubal muscle is inserted into the mucosa of the auditory tube. Often, inside the palatine aponeurosis there is a minor elevator muscle of the soft palate, which originates from the hook of the sphenoid bone.

Teeth. The number of teeth and their position are subject to considerable variability. Between the crown and root of medial incisors there is often a thickened enamel ring, which forms from the pressure of the antagonist tooth. On the internal surface of upper lateral incisors, near the back of their root, there is sometimes a tubercle. The canines (especially inferior) are often twisted about their axis and turned outward. In rare cases the canine may not erupt. Sometimes the canines develop after the adjacent teeth and, due to a lack of space, grow sideways. The premolars may be partially or completely absent. The number of roots of molars may vary. Often their roots converge together or diverge into different directions. Often the upper molars (especially second molars) have additional masticatory tubercles. The third molars (wisdom teeth) may not erupt, or may appear only after age 30. Sometimes there are accessory teeth, which are situated on the side of the gingiva. There are many possible variations of occlusion.

Tongue. Very seldom the tongue is absent (aglossostomia). The foramen cecum is absent in 7 percent of cases. The apex of the tongue may be cleft into two or three tips. There may be accessory muscles, such as the triticeoglossal muscle, which originates on the cartilago triticea of the larynx. There may also be an additional middle longitudinal muscle, accessory cornuoglossal or auriculoglossal muscles.

Cheeks. The size of the adipose body is highly variable, as well as the configuration of the cheek, thickness of the buccinator muscle and its origin and insertion.

Major salivary glands. There may be an accessory parotid gland near the anterior margin of the masseter muscle. Its excretory duct may open separately, but more often connects with the parotid duct. There may also be additional glandular lobules near the submandibular gland, by the lateral edge of the geniohyoid muscle. Sometimes there are additional sublingual glands. The number of small sublingual ducts varies between 18 and 30.

Pharynx. Very seldom the pharynx may be absent. One of its parts may have a narrowing or be interrupted. Sometimes there is a fistula between the pharynx and outside (branchiogenic fistulae), which correspond

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to unclosed branchial fissures. These fistulae may open by the posterior edge of the sternocleidomastoid muscle, above the sternoclavicular joint or near the mastoid process of the temporal bone. There are variations of the pharyngeal muscles. The inferior constrictor muscle of the pharynx has accessory fascicles, which extend from the trachea. In 4 percent of cases there is a ligamentopharyngeal muscle, which originates on the lateral thyrohyoid ligament and is inserted into the median or inferior pharyngeal constrictor. In 60 percent of cases there is a cricopharyngeal muscle, which sometimes gives an origin to the elevator muscles of the thyroid gland, which stretches to the right and left lobes of this gland. There are often muscle fascicles stretching between the middle constrictor of the pharynx and the intermediate tendon of the digastric muscle. The pharyngobasilar fascia is sometimes partially or completely replaced by an unpaired muscle, which attaches the pharynx to the skull. The stylopharyngeal muscle may be partially doubled. It often combines with accessory fascicles, which originate from the mastoid process (mastoidopharyngeal muscle) or on the surface of the occipital bone (occipitopharyngeal muscle). In the vault of the pharynx the mucosa may form one or several recesses 1.5 cm long and 0.5 cm wide (pharyngeal bursa). The pharyngeal bursa may be communicated with the pharyngeocranial canal.

Oesophagus. In rare cases the oesophagus may be absent; there may be closure of its lumen (atresia) or congenital diverticula. A very rare condition is a doubling of the oesophagus or existence of fistulae into the trachea. Occasionally there are fistulae from the oesophagus, which open on the skin of the lower neck region (near the front edge of the sternocleidomastoid muscle). The development of the muscularis of the oesophagus may vary. The transition between smooth and striated parts of the muscle layer may be located in different parts of the oesophagus. There may be bronchoesophageal or pleuroesophageal muscles. In 30 percent of cases in the lower section of the posterior mediastinum, behind and to the right of the oesophagus, there is a paraesophageal bursa 1.65–4 cm long. In 10 percent of cases the aorta and oesophagus pass through the diaphragm in a common foramen. The direction of the oesophagus, its flexures, extent and size of its constrictions are considerably variable.

Stomach. Very rarely the stomach is absent or doubled. There may be partial of complete transverse strangulations of its lumen, which may vary in shape and size. More often they are situated near the pyloric region. There are variations in thickness of the muscular layer of the stomach and the number and distribution of its glands.

Small intestine. There are many variants of shape and position of the duodenum. Aside from the typical horseshoe shape it may be shaped like a full or partial ring. Sometimes it lacks the horizontal or the descending part.

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