HUMAN ANATOMY – VOLUME 1

forms an endoblastic vesicle (yolk sac). The embryonic disk («body») is situated where the amniotic cavity connects to the yolk sac. During this stage the embryo has the shape of a bilaminar disk made up of an external (ectoderm) and internal (endoderm) embryonic sheets. The ectoderm faces the amniotic cavity, and the endoderm adjoins the yolk sac. At this stage it is possible to define surfaces of the embryo. The dorsal surface adjoins the amniotic cavity and the ventral surface contacts the yolk sac. The trophoblast cavity around the amniotic and yolk vesicles is loosely filled with cells of extraembryonic mesenchyme. By the end of the 2nd week the length of the embryo is approximately 1.5 mm. During this stage the bilaminar embryonic disk thickens in its posterior (caudal) part. This is where the axial organs (chorda, neural tube) will later develop.

During the third week of development the formation of the threelayered embryo takes place. Some cells of the ectoderm migrate toward its posterior end, forming a cell cord called the primitive streak. In the anterior (head) section cells of the primitive streak grow and divide faster, which results in the formation of the primary nodule (Hensel’s nodule). The primitive streak determines the bilateral symmetry of the body. The location of the primary nodule marks the cranial (head) end of the embryo.

Later on, the cells of the primitive streak and primary nodule grow in between the ectoderm and endoderm. This creates the middle embryonic layer — the mesoderm. Cells of the mesoderm, which stay between the two layers of the embryonic disk are called intraembryonic mesoderm; cells that have migrated outside the disc make up the extraembryonic mesoderm.

Part of the mesoderm cells within the primary nodule grows particularly actively, forming the cranial (chordal) process. This process penetrates between the external and internal layers up to the caudal end, forming the chorda (notochord). The cranial part of the embryo grows faster than the caudal end. At the end of the 3rd week a longitudinal strip of actively dividing cells (neural plate) appears within the ectoderm in front of the primary nodule. This plate then forms a longitudinal fold — the neural sulcus. As the sulcus deepens, its edges thicken, converge and grow together, forming the neural tube. The ectoderm closes over the neural tube and ‘detaches’ from it. Later on, the entire nervous system will develop out of the neural tube.

Also during this period a finger-shaped outgrowth — the allantois, forms by penetrating out of the posterior part of the endoderm into the extraembryonic mesenchyme (the so-called amniotic stalk). The allantois does not carry out any specific functions in humans. Next to the allantois, umbilical blood vessels grow through the amniotic stalk towards the chori-

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onic villi. By the end of the 3rd week, the human embryo has the appearance of a trilaminar plate or disk. In the region of the ectoderm a neural tube can be distinguished with the chorda somewhat deeper. Thus, the axial organs of the human embryo are formed.

The fourth week of embryonic development. The trilaminar embryonic disk begins to bend in transverse and longitudinal directions. It becomes convex and its edges are separated from the amnion by a deep groove called the truncal fold. The embryo turns from a flat disk into a volumetric structure. Ectoderm covers the embryo from all sides.

The endoderm inside the embryo rolls up into a tube, forming the embryonic rudiment of the future gut. A narrow opening, which connects the embryonic gut with the yolk sac, later turns into the umbilical ring. The endoderm forms the epithelium and glands of the gastrointestinal tract. The ectoderm forms the nervous system, the dermal epithelium and its derivatives, the epithelial tegument of the mouth, the anal part of the rectum and the vagina. The mesoderm develops into internal organs (excepting the derivatives of the endoderm), the cardiovascular system, organs of the locomotive apparatus (bones, joints, muscles) and the dermis.

The embryonic (primary) gut is at first closed at both ends. Then invaginations appear in the anterior and posterior ends of the embryo, marking the oral cavity and anal fossa. The primary gut cavity is separated from the oral fossa by the double anterior (oropharyngeal) membrane. The gut and the anal fossa are separated by a cloacal (anal) membrane. The anterior (oropharyngeal) membrane breaks in the 4th week of development, while the posterior (anal) membrane breaks in the 3rd month.

As a result of its curving the embryo becomes surrounded by amniotic fluid, which makes up a protective environment and prevents possible damages — primarily mechanical (concussions) — to the embryo. The yolk sac is much slower in growth. In the 2nd month of intrauterine development it appears as a small saccule, and later is reduced completely. The ventral (body, abdominal) stalk, which contains blood vessels that connect the embryo with the placenta, begins to be called the umbilical cord.

Starting with the end of the 3rd and throughout the 4th week the differentiation of the mesoderm continues. Its dorsal part, located at the sides of the chorda, forms paired protrusions called somites. These somites become segmented, meaning divided into paired sections. Because of this, the dorsal part of the mesoderm is called segmented. Segmentation of somites takes place gradually from the front backwards. On the 20th day of development the third pair of somites forms, by the 30th day there are 30 of them, and on the 35th day there are 43–44 pairs. The ventral part of the mesoderm is not

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segmented. It forms plates on either side of the primary gut (Fig. 31). A medial (visceral) plate adjoins the endoderm (primary gut) and is called the splanchnopleura. The lateral (outside) plate adjoins the wall of the embryo (the ectoderm) and is called the somatopleura. The splanchnoand somatopleurae form the epithelial covering of the serosae (mesothelium), as well as the lamina propria serosae and the tela subserosa. The mesenchyme of the

Fig. 31. Transverse section of an embryo. splanchnopleura forms all layers of the digestive tube

except for its epithelium, which derives from the endoderm. The endoderm forms glands of the esophagus, stomach and intestine

as well as the liver with the biliary ducts, the pancreas and epithelial linings of the respiratory organs. The space between plates of the unsegmented mesoderm turns into the celom.

The mesoderm on the border between the somites and the splanchnopleura forms nephrotomes, which form the duct of the mesonephros. The dorsal part of the mesoderm (somites) forms the bones and muscles of the embryonic trunk. The ventromedial region of the somites (sclerotome) forms into bones and cartilage of the axial skeleton — the vertebral column. To the outside of the sclerotome lies the myotome, which forms the skeletal muscles. The dorsolateral region of the somite is called the dermatome, which develops into connective tissue base of the skin, or the dermis.

During the 4th week, in the cranial part of the embryo the ectoderm forms primitive ears (first the aural fossae, following by the aural vesicles) and eyes (future lenses). At this time there is a transformation of the visceral regions of the head, which are grouped around the oral cavity in the form of the frontal and maxillary processes. Caudal of the latter the contours of the mandibular and hyoid visceral arcs can be seen.

On the ventral surface of the embryonic trunk there are several heightenings, namely the cardiac and hepatic tubera. The dent between these

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tubera indicates the place of formation of transverse septa — rudiment of the diaphragm.

Caudal of the hepatic tuber is the abdominal stalk, which connects the embryo with the placenta (umbilical cord).

The period between the 5th and 8th weeks is the beginning of organ (organogenesis) and tissue (histogenesis) development. This is the period of early development of the heart and lungs, elaboration in the structure of the gut, formation of the visceral and branchial arches and capsules of sensory organs. The neural tube becomes dilated in the cranial end (future encephalon). At days 31–32 (5th week, length of embryo is approximately 7.5 cm/ finger-like rudiments, ‘buds’, of the hands form. By day 40 rudiments of the future legs appear.

During week 6 rudiments of the outer ears become noticeable. Starting at the end of weeks 6–7 fingers of the hands, and later toes become visible. By the end of week 7 eyelids begin to form, which makes the eyes appear more defined. By the 8th week formation of organ rudiments becomes complete.

Starting on week 9, or month 3, the embryo begins to appear like a human and starts to be called a fetus. Beginning in third month and during the entire fetal period growth and further development of organs and body parts take place. Differentiation of external sex organs also begins at this time. Rudimentary nails form, and at the end of month 5 eyebrows and eyelashes start to be visible. During month 7 the eyelids open. At this time fat begins to accumulate in the subcutaneous tissue. In the tenth month the fetus is born.

After birth the child grows quickly, his mass, length and surface area of the body increase likewise. A human being grows during the first 20 years of life. For men growth of body length ends at approximately 18–22 years; for women — at 18–20 years. Up to 60–65 years of age body length stays almost constant. During old age (after 60–70 years), however, the length of the body decreases by 1–1.5 mm each year due to the increase of curving in the vertebral column, thinning of intervertebral disks and flattening of foot arcs.

In the course of the first year of life the height of a child increases by 21–25 cm. In the beginning of the second childhood period (8–12 years) height increases at a rate of 4.5–5.5 cm per year and later speeds up. During adolescence (12–16 years) body length increases yearly by 5.8 cm for boys and 5.7 cm for girls. The most intensive growth period for girls is between 10–13 years and for boys — during adolescence (13–16 years). After that growth slows down. The mass of the human body at 5–6 months after birth doubles, and by two years of age increases by 4 times. The

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Fig. 32. Modification of body’s proportions within the process of growing (acc. to A. Andronesku).

KM — median line; according to vertical axsis (numbers of right side) — correspondence between bodies of childs and adults; superior horizontal axsis — correspondence between length of head and body.

maximum yearly mass increase for girls is during the thirteenth year, and for boys during the fifteenth year. Weight continues to increase up to 20– 25 years and then remains stable up to age 40–46. It is considered important and physically justified to keep body mass within limits of its values at age 19–20.

During the recent 100–150 years a speeding up (acceleration) of morphological and functional development of the whole body has been noted in children and adolescents. Thus, the body mass for newborns has increased over the century by 100–300 grams, and for one year olds by 1500–2000 grams. Body length during the second childhood period and adolescence increased by 10–15 cm, and for adult males — by 6–8 cm. The time period during which height increases shortened. In the end of the XIX century growth continued until 23–26 years. In the end of the XX century height increases in men until age 20–22, and in women until age 18–20. Dentition of both milk and permanent teeth has accelerated. Mental development and sexual maturation proceed faster. In the end of the XX century, compared to its beginning, the average age of menarche for girls lowered from 16.5 to 12–13 years. The age of menopause for an adult shifted from 43–45 to 48–50 years.

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During the postnatal period of growth each age has its morphological and functional characteristics (Fig. 32).

KM-medial line; along the vertical axis, numbers show an accordance of human body portions (concerning children and grown-ups), the upper horizontal axis shows the length concernment of head and body.

A newborn has a relatively large round head, short neck and chest, a long abdomen, short legs and long arms. The cerebral part of the skull is comparatively larger than the facial part. The shape of the thorax is barrellike. The vertebral column does not have curves. The internal organs are relatively bigger than those of an adult. Thus, the mass of the liver for a newborn makes up 1/20 of total body mass, while in adults it is 1/50. The length of the intestine is twice as long as body length, whereas in adults it is 4–4.5 times as long. The mass of the brain in a newborn makes up 13– 14% of the body mass, whereas in an adult it is only approximately 2%. The thymus and adrenal glands are especially large.

During the suckling age (10 days — 1 year) the baby’s body grows rapidly. At about 6 months teething of milk teeth begins. During the first year the size of some organs reaches adult size (the eye, inner ear, central nervous system). During the first year of life there is a quick development of organs of the locomotion apparatus, the digestive system and the respiratory system.

In early childhood (1–3 year) primary dentition ends. Psychic development, speech and memory abilities progress quickly. The child starts to become oriented in space. In the end of this period secondary /permanent/ dentition begins. Because of fast development of the brain, the mass of which by now reaches 1100–1200 g, mental capabilities develop quickly, as well as long term ability for recognition and orientation in time and days of the week.

During the second childhood period (3–12 years) growth in width predominates again, although growth in length, which in this period is greater for girls, also increases. The psychic development of children progresses.

Orientation in months and days of the calendar develops. Sexual maturation starts, beginning earlier for girls, due to an increase in female hormone secretion. In girls at the age of 8–9 the pelvis and hips begin to widen, sebaceous gland secretion increases and hair appears on the pubis. In boys of 10–11 year the larynx, testes and penis begin to grow.

In adolescence (12–16 years) sex organs develop quickly and secondary sexual characteristics strengthen. For girls the amount of hair on the pubis increases and hair appears in the axillary fossae. Sexual organs and gonads increase in size. The basic pH of vaginal secretions becomes

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LOCOMOTION APPARATUS

There are a lot of different regions in human body (parts).

Two of the most important functions of the body are movement and holding itself in determined position. These functions are carried out by the support and locomotion apparatus, which is made up of active and passive components. The passive component includes bones (hard skeleton), which support muscles and different organs, and joints (Fig.33). The active component of the locomotive system includes muscles, which by contracting bring bone «levers» to movement. The human body also has a soft skeleton (framework), which helps to keep organs near bones. The soft skeleton consists of fascies, ligaments, fibrous capsules and other structures.

STRUCTURE OF BONES

Bone tissue of the hard skeleton, which consists of the vertebral column (spine), the breastbone and ribs (bones of the trunk), skull and bones of upper and lower extremities. The skeleton carries out functions of support, movement, resilience, protection and also serves as a depot for various salts (mineral substances).

The function of support consists in the skeleton providing a hard bone and cartilage framework, to which soft tissues and many organs are attached. The movement function is realized by means of joints, which can be brought to move by muscles. The function of resilience consists in reducing and softening concussions due to movement through the presence of special anatomical structures (construction of the foot, cartilage lining between bones, etc.). The protective function is carried out by providing bone casing for the brain and sensory organs (cavity of the skull) and for the spinal cord (spinal canal). Bones also contain bone marrow, which is the source of blood and immune system cells, and are a depot for mineral salts. A bone contains minute quantities (up to 0.001%) of more than 30 different chemical elements (Ca, P, Mg, etc.).

The skeleton contains an average of 206 bones. Among them there are 36 unpaired and 85 paired bones. The mass of a «living» skeleton is 11 percent of total body weight for newborns and 9–18 percent for children of other ages. In adults this correlation stays at approximately 20 percent throughout the whole life. During old age the mass of the skeleton decreases.

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Fig. 33. Human skeleton. Anterior aspect.

1 — cranium; 2 — vertebral column; 3 — clavicle; 4 — rib; 5 — sternum; 6 — humerus; 7 — radius; 8 — ulna; 9 — capral bones; 10 — metacarpals (1—V); 11 — phalanges of fingers of hand; 12 — ilium; 13 — sacrum; 14 — pubis; 15 — ischium; 16 — thigh bone; 17 — patella; 18 — tibia; 19 — fibula; 20 — tarsal bones; 21 — metatarsals; 22 — falanges of fingers of foot.

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For research purposes and learning material bones can be macerated (this is attained by degreasing, bleaching and then drying them).

CLASSIFICATION OF BONES

The classification of bones is based on three principles: the shape and structure of a bone, its development and its function. Bones are divided into long (tubular), short (spongy), flat (broad), irregular (mixed) and pneumatic (Fig. 34).

Long bones have a tubular shape and form the basis of limbs. They act as long bony levers. Their diaphysis is usually cylindrical or trihedral (Fig.35). The thickened ends of long tubular bones are called epiphyses. Epiphyses have articular surfaces covered by cartilage, which serve for joining neighboring bones. Between the diaphysis and epiphysis is the part of the bone called metaphysis. This region corresponds to cartilage that has ossified during the course of postnatal development. The metaphysis has a cartilage zone by means of which the bone grows lengthwise. Tubular bones can be subdivided into long (branchial, femoral, etc.) and short bones (metacarpus, metatarsus).

Spongy bones are found in parts of the skeleton, where considerable mobility of bones is combined with great mechanical durability (carpal and tarsal bones). This group also includes sesamoid bones, which lie within

Fig. 34. Types of bones.

I-pneumatized bone (ethmoidal bone); II — long bone; III — flat bone; IV — shot (spongy) bones; V — irregular bone.

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