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Fig. 1. General anatomical plan of the body wall.

Fig. 2 Representative diagram of the relationship between viscera, coelomic cavity and the body wall.

6. Body Wall and Coelomic Cavity. This chapter deals with some of the key points in the development and gross anatomy of the general plan of the body. The trilaminar embryo that we have been describing is the forerunner of the plan of the head, neck and the trunk. The limbs develop as outgrowths of the wall of this part. The head is a rather specialised region, and along with the neck, involves some special developmental components and processes. The trunk houses the major parts of many organ systems. We focus our attention on the trunk.

The thorax and the abdomen, the two parts of the trunk, have a cavity and a wall. We shall begin with a very simple, anatomical plan for the body wall. A typical cross section of the abdomen is a good place to begin (Fig. 1). Let us not worry about the internal organs to start with. On the posterior side we can recognise a vertebra. All around, the body is covered by the skin. This includes the epidermis and the dermis. The epidermis is ectodermal in origin; the dermis is connective tissue of mesodermal origin. Deeper inside is a layer of muscle. Note that part of this muscle layer is posterior to the vertebra; the larger part is anterior.

These structures – the skin, muscle and associated connective tissues form the body wall. Typically, the aorta, just in front of the vertebral column, is the major blood vessel supplying the wall and the viscera. The body wall surrounds a cavity. It is lined by an epithelium, which, along with its supporting connective tissue forms a membrane. The membrane produces a watery fluid. This gives it the name serous (“watery”) membrane. The fluid fills the cavity. Next, we see how a representative organ, say intestine, is located in relation to the cavity. In fig. 2 the intestine (X) is seen in the connective tissue outside the epithelium of the cavity.

Notice that the organ seems to push one side of the membrane of the cavity (arrow in ‘A’). The organ is outside the cavity – the cavity ‘wraps around’ the organ. Finally, the organ is almost completely surrounded by the serous membrane. The membrane now has three distinct regions continuous with each other (B). The part of the serous membrane adhering to the organ is the visceral layer, the part lining the body wall is the parietal layer. Connecting the two is a double-layered fold carrying a blood vessel to the organ. In the abdomen this fold is known as the mesentery. This description makes this situation easy to understand. During embryonic development the organ does not quite ‘push’ into the cavity.

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Fig. 3. Three successive stages of the trilaminar embryo. Part ‘1’ is to get our bearings : Note ectoderm (Ect), endoderm (End) and mesoderm (unlabelled). The notochord defines the axis. A special area of the ectoderm forms the neural plate. The three short, unlabelled arrows indicate that the mesoderm is being subdivided into three columns. In ‘2’, the neural plate has deepened into a groove. But more important for us, the three divisions of the mesoderm are labelled : P – paraxial, LP – lateral plate, I – intermediate. ‘3’ is unlabelled for clarity. Follow the divisions and note that cavities in the lateral plate have merged into one on each side. Also note that in ‘3’, the neural tube and neural crests are fully formed.

Fig. 4. Somites

Let us now see how this relationship comes into existence. We need to go back to the trilaminar embryo.

Embryology : Body wall and coelomic cavity. We have seen earlier a series of cross sections of the trilaminar embryo, showing the paraxial, intermediate and lateral plate mesoderm. We have also seen that the lateral plate mesoderm splits into two layers separated by the coelomic cavity.

Fig. 3 recapitulates the divisions of the mesoderm and the formation of the neural tube. In these figures, some details have been simplified or omitted. You will notice that the ectoderm and endoderm are continuous with layers outside the body of the embryo proper. We can afford to ignore these details, at least for the time being. Similarly, the mesoderm also continues as extra-embryonic (outside the embryo) mesoderm. We need not worry about this as well. The cavity that appears in the lateral plate mesoderm is called the coelomic cavity. At this stage we are going to focus on the paraxial mesoderm and the lateral plate mesoderm. We shall study the intermediate mesoderm with the urogenital systems. Paraxial mesoderm is like a column by the side of the notochord and the neural groove/plate. This column of paraxial mesoderm splits into brick-like blocks – this the starting point for the segmental structure of the body. These blocks, called somites, are shown in a drawing of the entire embryo in fig. 4. In this figure, the axis of the body runs between the two arrows and the coloured portions are the somites. Somites appear in sequence from the head end to the tail end of the embryo. The other two divisions of the mesoderm, intermediate and lateral plate, are not highlighted in this drawing. Each somite gives rise to structural components of a segment (slice) of the body. As the first indication of this, a somite differentiates into two parts : sclerotome and dermo-myotome. The sclerotome will give rise to a part of the vertebral column. (The term literally means the “hard cut” – bone is hard!) The dermo-myotome differentiates further into two parts : a dermatome and a myotome. The dermatome (= “skin cut”) gives rise to the dermis of the segment. The myotome forms the muscle of the segment. This development and its consequences are explained on the next page.

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Fig. 5 continues from fig. 3. L-p is the lateral plate mesoderm and the asterisks mark the cavity. The two parts of a somite are indicated by the split arrow and the two labels ‘Scl’ and ‘DM’. In further descriptions in this chapter, we shall focus on the myotome. In this and further figures, there appears a fair amount of ‘empty space’ between the ectoderm and endoderm, around the somites. This is for clarity –in reality there is no such empty space!

Fig. 6. Folding of the trilaminar embryo. Analogies cannot be perfect, but they help!

Fig. 7. Lateral folding in progress. Compare with fig 5 above.

How does a flat embryo transform itself from a flat plate to a three dimensional structure with cavities? It simply folds up! A little paper exercise will go a long way in understanding this. We are making a four-flap envelope from a cut piece of paper. Mark one end as the head end, one as the tail end and two side flaps. When they are folded as shown in fig. 6, you have a structure with a cavity. In this example, on the left side we are looking at the endodermal side of the embryo. (The ectoderm is on the reverse). So the cavity of the envelope is bounded by the endoderm. When we fold the flaps, we can see only the ectodermal side all around. And if the fully formed envelope is the embryo, we are looking at the anterior (ventral) surface in the view in fig. 6. If you try this little game (and I strongly recommend you do!), it would be worthwhile darkening one surface of the sheet of paper with a soft pencil, so that the ectodermal and endodermal surfaces can be distinguished. The right and left flaps are lateral plate mesoderm, and each has a cavity within itself (see fig. 5). Realise that the flaps do not fold ‘one-after-the other’! They all converge together, so that there is an opening in the belly region of the embryo – this opening gradually closes as the embryo grows. At this stage let us consider only the lateral folds, because we shall soon look at transverse sections of the trunk. We shall discuss the head and tail folds when we describe the development of the heart, and later, that of the digestive system. Now let us turn to the transverse sections.

Observe and describe the changes seen in fig. 7 to yourself : The sclerotome begins to form segmental bones, which will develop into vertebrae. The body of a vertebra has surrounded a part of the notochord. The part of the notochord trapped in the body of a vertebra disappears. The myotome begins to spread. The dermatome is not shown to keep the picture simple, but it also spreads under the ectoderm of the segment. The lateral plate mesoderm has split into two layers (seen even in the earlier stage). Note the cavity, marked by the asterisk, between the two layers.

“L-p : V”. This layer goes with the endoderm and is closely related to the visceral structures. Therefore it is called the visceral layer. “L-p : P”. This layer goes along with the ectoderm. Deep to the dermis, it forms the connective tissues of the body wall (“parietes”). Therefore this layer is called the parietal or “somatic” layer. Though these two layers are continuous with each other at the dorsal ends, for a major part they develop different characteristics. Keep this in mind. Finally, note that the dotted white line demarcates two parts of the myotome. One part is dorsal to

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Fig. 8. (Left) Representative diagram of the relationship between viscera, coelomic cavity and the body wall. Above : Fig. 2B repeated for comparison.

Fig. 9. Spinal nerve and body wall musculature.

the axis (notochord) – this is the epimere (= “portion above”) and forms muscles dorsal to the axis (epaxial muscles). The other, ventral part, is the hypomere, and gives rise to hypaxial muscles. You may choose to omit the terms epimere and hypomere and simply remember the final products, epaxial and hypaxial muscles. The lateral folds are completed when the two sides meet each other in the midline. This is shown in fig. 8. Note that the epaxial muscle mass is on the dorsal side of the vertebra. The hypaxial mass spreads out over the rest of the wall. The visceral layer of the lateral plate mesoderm is now seen to encircle the endodermal tube. As mentioned before, it forms the connective tissue and muscle of the wall of the digestive tube. From its early appearance, the coelomic cavity is lined by an epithelial layer, formed by a single layer of mesodermal cells. As mentioned earlier, this epithelium, with its supporting connective tissue, forms the serous membrane. In the abdomen, this is the peritoneal membrane. Compare these features with the gross anatomical plan shown in fig. 2B, repeated here. An extremely important concept needs to highlighted here. Comparing fig. 2B with fig. 8, you can see that the coelomic epithelium of the visceral layer and the mesentery develops with the visceral layer of (lateral plate) mesoderm. That of the parietal layer develops from the parietal layer of mesoderm. The visceral layer of a serous membrane (peritoneum in this section of the abdomen) becomes an integral part of the organ. The parietal layer becomes a part of the body wall. This has an important bearing on the nerve supply and nature of sensation of these peritoneal layers. Pain from the visceral layer, like pain from an organ itself, is ‘referred’ to appropriate body segments, depending on their sympathetic nerve supply. Pain from the parietal layer is felt like pain arising in the body wall tissues and is localised to the area. This concept is explained in the section on the autonomic nervous system (Chapter 6). The discussion so far has introduced a number of key concepts and words. These ara summarised at the end of the chapter. Make sure you understand them well.

The next question we ought to ask is : How does this developmental plan correlate with the anatomy of the spinal nerves? A spinal nerve is a mixed nerve. In fig. 9, its sensory (dorsal) and motor (ventral) roots are shown attached to the spinal cord. They unite to form the spinal nerve, which is a short trunk between the two vertical lines. The nerve divides into two primary rami (= branches), both of which are mixed nerves. Note that the dorsal ramus enters the epaxial muscle mass and then continues to the overlying skin of the segment. The ventral ramus enters the hypaxial muscle mass. It supplies this muscle mass and also the remaining skin over the segment. This concept is also elaborated and correlated in the discussion on spinal nerves and the thoracic and abdominal walls. In later chapters, we shall also examine this concept in the context of the innervation of the limbs.

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Fig. 10. Distribution of mesoderm.

X

C D

* *

Y Y

Fig. 11. Thorax – bones.

2 3 8

The coelomic cavity in the thorax. The coelomic cavity in the thorax is a little different in appearance. We need to go back to a picture from early embryology, redrawn here as fig. 10. The transverse sections we have been studying so far are roughly along the line C-D in this figure. Note how the lateral plate mesoderm has spread at the head end of the embryo, in the form of a horseshoe. The coelomic cavity within this part also has a horseshoe-shaped curve. The light coloured area within the curve is where the ectoderm and endoderm are in contact, an area called the oral membrane. This entire portion folds down as the “upper flap of the envelope” in our earlier analogy. When this flap (the head fold) turns down, the region marked ‘X’ and the horseshoe comes to lie in the thorax! The region ‘X’ of the coelomic cavity forms the pericardial cavity – the heart itself develops from a small area of mesoderm under ‘X’. the parts immediately lateral (Y) form the pleural cavities around the lungs. The lungs develop from outgrowths of the digestive tube. These topics are the subjects of later chapters. The key concepts we are driving home here are : The heart is located around the midline, and so is the pericardial cavity. The lungs are lateral and so are the pleural cavities.

There are communications between the pleural cavities and the peritoneal cavity of the abdomen at the points marked by asterisks in this picture. These “pleuroperitoneal canals” normally disappear. Similarly, there are communications between the pericardial and pleural cavities, which also disappear normally.

Body Wall : Regional Anatomy

The thoracic and abdominal walls are built to a common plan. This common plan was shown in fig. 1; and we have the developmental basis of this plan. However, the detailed structure reveals some interesting structural adaptations. The thoracic wall needs to move during respiration. The muscular wall of the thorax is interrupted by the ribs. In the abdomen, the wall functions mainly to regulate and vary the pressure within the cavity. We shall discuss these in that order.

The thoracic wall : bony framework. (Fig. 11) Running along the midline of the back is the thoracic vertebral column. Next, there are twelve pairs of ribs. The Latin word for a rib is ‘costa’, and most anatomical terms pertaining to ribs include the word ‘costal’. Ribs are attached posteriorly to the vertebrae. They curve around the thorax and are attached to a flat bone in front, the sternum. For the lower ribs, this attachment is indirect (see below). Their sizes increase progressively until about the 8th or 9th rib, and then they become shorter. The last two ribs are attached only to the vertebrae. Note that the first 6 or 7 ribs are attached individually to the sternum through a piece of hyaline cartilage. Beyond this, each rib until the tenth, is attached to the cartilage above. In fig. 11, ribs 2, 3 and 8 are indicated by arrows.

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Fig. 12. A typical rib.

Fig. 13. A : Pectoral muscles belong to the upper limb! B : Pectoralis major cut to show ribs (arrows) and intercostal spaces.

A B

Despite some differences among the ribs, we can describe a common pattern in their features. A typical rib has a ‘head’, a short neck, a tubercle and a long curved body. (Fig. 12). The head, neck and the tubercle are posterior. The anterior end is joined to a piece of hyaline cartilage (costal cartilage), which in turn connects to the sternum, directly or indirectly as mentioned above. We come across terms like ‘true’ ribs (ribs 1 to 6, attaching directly), false ribs (7 to 10) and ‘floating ribs (11 and 12). These terms do not really add to our understanding of these structures! It is easier to understand the pattern by rib numbers. The continuous line formed by costal cartilages 6 to 10 and the tips of 11 and 12 is termed the costal margin, an important landmark in the anatomy of the thorax and the abdomen. The sternum. This is a flat bone, anteriorly located in the midline. Shaped like a dagger, it has three parts. The upper, handle-like part is the manubrium (the term means a handle!); the middle part is the body, and the lower part is called the xiphoid (“sword-like”) process, also known as xiphisternum. In fig. 11, locate the parts of the sternum by the initial letters of their names. Notice that the second costal cartilage is attached to the junction between the manubrium and the body. Observe the unattached tip of the 11th rib below the 10th ; the tip of the 12th rib is hidden. The head and the tubercle of a rib form movable (synovial) joints with the vertebral column. The tubercle articulates with the transverse process of numerically the same vertebra. The joints of the head with the bodies are complex; typical ribs articulate with bodies of two vertebrae and the disc in between. Soft tissues of the thoracic wall. The thoracic wall is formed by the skeletal elements described above, with the associated muscles. However, muscles that belong to the upper limb are also attached to these bones. Such muscles are not truly muscles of the thoracic wall. We shall describe them with the plan of the upper limb. Here they are mentioned because

they are the first muscles seen in a dissection of the thorax. These include the muscles of the pectoral region. The term ‘pectoral’ comes from the word ‘pectus’, meaning the anterior part of the chest. Fig. 13A shows the large (major) pectoral muscle and the outline of the small (minor) pectoral muscle deep to it. The manubrium, body and the xiphoid process are indicated by appropriate letters, and the costal margin as well (“Cm”). The clavicle (collarbone) is indicated by a ditted line (C). In fig. 13B, a part of the right pectoralis major is cut to reveal ribs under its cover.

The thoracic wall shows a repeating pattern if ribs and ‘spaces’ between the ribs. The spaces between the ribs are the intercostal spaces, occupied by muscles, blood vessels and nerves. The term intercostal (= between ribs) is used to name the muscles, blood vessels and nerves between two ribs. The muscles of the thoracic wall proper are in three layers. The outermost muscle is the external intercostal. Deep to it is the internal intercostal, forming the middle layer. Deeper still is the innermost intercostal. These muscles are confined to an intercostal space – thus, there are as many pairs of these muscles as there are intercostal spaces. In the anterior and posterior parts of the wall, the innermost layer is modified to form additional separate muscles called the transversus thoracis and the subcostalis.

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Fig. 14. Arrangement of the intercostal muscles. A : An entire space. B : A vertical section through the space.

X

Y

A B

Fig. 15. Azygos venous system.

The fibres of the external and internal intercostals run roughly at right angles to each other. The innermost intercostal follows the same direction as the internal. The curvature of the ribs can cause some confusion in understanding the directions – you will need to check these in laboratory specimens. Fig. 14A shows this arrangement. The innermost intercostal is not shown in this picture. Note that in the lower part of the picture, a part of the external intercostal is cut out to show the deeper, internal muscle. The light homogenous areas (X and Y) the muscles are replaced by connective tissue membranes. On the right (B) is a vertical section through an intercostal space, showing these muscles and the intercostal nerves and blood vessels. Note that the main neurovascular bundle (nerve and blood vessels) runs between the internal and innermost intercostal muscle, in a groove along the length of the rib. Nerves and blood vessels. The nerves of the thoracic wall are intercostal nerves. These are ventral rami of thoracic spinal nerves. Each intercostal space represents one segment of the body wall. The arteries are largely branches of the thoracic aorta. These branches are called posterior intercostal arteries. They supply intercostal spaces 4 to 11. Corresponding arteries for the upper three spaces come from the subclavian system. In addition to the posterior arteries, there is a vertical artery (internal thoracic) running by the side of the sternum which gives small anterior intercostal arteries. Understand that the nerves and the larger arteries come from the posterior side! The major blood vessels (aorta, subclavian arteries etc) will be studied with the thoracic organs in a separate chapter.

An artery running along the border of the sternum, the internal thoracic artery, also supplies blood to the anterior part of the thoracic wall. This is an interesting artery, to be mentioned again with the abdominal wall. The veins of the thoracic wall also follow a pattern. Posterior intercostal veins from the fourth to eleventh spaces join two veins by the side of the vertebral column. On the right side this vein is continuous, and is called the azygos vein. On the left side it is broken up into two parts, each of these is the ‘hemiazygos’ (= ‘half azygos’) vein. Both hemiazygos veins join the azygos vein, which in turn empties into the superior vena cava. Veins from the upper three spaces join other larger veins in the region. This factual detail underscores the basic plan of the body wall veins. Study fig. 15. You will notice that segmental veins join two parallel channels. These two long, parallel channels were connected to each other across the midline. During development some parts have disappeared (dotted portions). The resulting picture appears complex, but the underlying plan is simple. More about the larger veins in the thorax in the chapter on the thorax! At this stage, the segmental pattern of the wall is the key concept.

Remember – there is no rib after the twelfth! The nerve below the 12th rib (after the 11th intercostal) is therefore called subcostal, so also the artery and the vein.

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Fig. 16. The diaphragm is dome-shaped.

Fig. 17. Bony elements of the abdominal wall.

Fig. 18. Abdominal wall.

The abdominal wall : Bony framework and overlap with the thorax. The thoracic and abdominal cavities are separated by a thin sheet of muscle, the diaphragm. The diaphragm is shaped like a dome. This creates a significant overlap between these two cavities and their walls. The diaphragm is a thin muscular sheet, attached all around the boundary (vertebral column, costal margin and xiphisternum) and converging on to a centrally located flat tendon. In fig. 16, note the lungs (L) in the narrow space between the ribs and the diaphragm and the abdominal cavity deep to these. The heart (H) sits on the central part of the diaphragm. Apart from understanding the anatomy, these facts are important when considering injuries to these parts of the body. A sharp object or a fractured rib can easily damage upper abdominal organs as well as the lungs.

A glance at fig. 17 will also show that, to be effective, the abdominal wall must reach at least as high as the level of the xiphisternum. At the lower end of the wall are the hip bones. The upper margin of the hip bone is the iliac crest (more about this with the pelvis); and anteriorly is the joint between the two pubic bones. Between the hip bones and the costal margins, the only bones are the vertebrae on the posterior side. Here some of the abdominal wall muscles are attached to the lumbar vertebrae through a layer of fascia. The lumbar vertebral column is strong yet flexible, with large vertebrae, their thick musculature and the surrounding fascia. Indeed, anatomically we describe this part separately as the “posterior abdominal wall”. It is convenient to study the posterior abdominal wall with abdominal organs like the kidneys.

The rest of the wall is anterior and lateral (anterolateral). This part is usually and conveniently called the “anterior abdominal wall” (fig. 18). At this stage we focus solely on the ‘anterior abdominal wall. The general pattern is : a covering of skin with superficial fascia just deep to it, a complex muscular layer, a connective tissue layer deeper still and parietal peritoneum.

Muscles. It is the muscle layer that demands great attention in the anterior abdominal wall. It is something like a pressure jacket. The muscles run in different directions to make this effective. The arrangement also leaves very few weak spots. The jacket maintains pressure inside the abdomen. Strong contraction can increase the pressure significantly. The greater part of the abdominal cavity is occupied by hollow organs of the digestive system. These organs undergo great changes in their sizes, depending their state of fullness. The muscles adjust their contraction to allow these changes. The urinary bladder is yet another organ that varies greatly in its size. During pregnancy in women, the volume of the uterus increases phenomenally. The muscular wall must also allow for these changes. In keeping with these functional requirements, the anterior abdominal wall has no deep fascia. However, the large muscular sheets have large aponeurosis – the toughness of the aponeurosis and the adjustment of muscle contraction effectively provides a dynamic yet strong wall. In the lateral part of the wall there are three muscular sheets. Their aponeuroses from the two sides meet in the midline in a line called the linea alba (= “white line”, so called because of its appearance – it is fibrous tissue). It forms a strong anchor for these muscles anteriorly, where there is no bone for muscle attachment. Close to the midline, the wall is further strengthened by a vertical strip of muscle.

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Fig. 19. The three lateral muscles of the abdominal wall. A: External oblique. Note : its upper limit, its large aponeurosis and the inguinal ligament. B : Internal oblique. This muscle is revealed when the external oblique is completely removed. Note its direction in contrast to the external oblique. Though it does not reach as high as the external oblique, this is a thick and strong muscle. C : Transversus abdominis as revealed after complete removal of the internal oblique. Note that in the lowest part of the abdominal wall, the directions of these muscles are similar.

Iliac crest

Inguinal ligament

The three lateral muscles are named by their location and the general direction of muscle fibres. Two of them have fibres tunning obliquely, the innermost has transverse fibres. Thus they are named external oblique, internal oblique and transversus respectively. Follow the description below with reference to fig. 19. The external oblique muscle. This large sheet reaches as high as the 5th

rib. The upper fibres are horizontal

and the lowermost (from rib 12) are almost vertical; but the general direction is downwards and medially. It is attached to a great part of the iliac crest. However, its large aponeurosis reaches the midline. Between the iliac crest and the pubic bone, the aponeurosis is unattached to bone and has a thick free margin called the inguinal ligament. Recall that the groin is called the inguinal region! The internal oblique. It is the thickest and the strongest of the abdominal wall muscles. Conventionally, its “origin” is described as being the lower end. It is attached here to the iliac crest and a large part of the inguinal ligament. The general direction of the fibres is upwards and towards the midline. It reaches the lower four ribs, and its aponeurosis reaches the midline. The lower end of the aponeurosis is a curved band of fibrous tissue called the conjoint tendon (“C”). “Conjoint”, because the transverses joins it. Also note the gap, indicated by an asterisk, between the internal oblique and the medial part of the inguinal ligament. Compare the general directions of the fibres of these two muscles in the two figures. A simple exercise will help you remember this : If you cross your forearms across your abdomen, they show the direction of the external oblique. If you bend your elbows and cross your forearms across the chest, you see the direction of the internal oblique. The transversus abdominis (7.22) is the deepest of the three muscles. As the name suggests, the direction of its fibres is transverse or horizontal. From a wide origin from the lower 6 ribs, indirectly from the lumbar vertebrae, iliac crest and the inguinal ligament. The fibres go towards the midline where the aponeuroses from the two sides meet.

Note that in their lowest parts, their fibres slope down. Deep to the transversus is a layer of connective tissue called the fascia transversalis. This fascia is rather thin, not easy to see in prosections. Diagrammatically, it is best shown in a transverse section. Rectus abdominis. Close to the midline is this vertical band of muscle. Note that in fig. 19 we have shown the aponeuroses of the three lateral muscles without the rectus abdominis, because it is hidden among the aponeuroses, with which it has a special anatomical relationship.

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Fig. 20. Rectus abdominis.

Fig. 21. Rectus sheath.

When the aponeuroses of the other muscles are dissected out, the rectus abdominis is seen as a vertical strip. (Fig. 20). Narrow at the lower end where it is attached to the pubic part of the hip bone, it broadens and reaches the cartilages of ribs 5, 6 and 7. A characteristic feature of this muscle is horizontal fibrous bands which divide the muscle into several parts. There usually three or four such “intersections”, but they are slightly variable. In a really well-developed rectus abdominis, the muscular portions bulge, and the fibrous bands are seen as grooves between the bulges – quite obvious even through the skin. This, then, is what creates the appearance of the ‘six-pack’! The next question we tackle is “How are these aponeuroses related to the rectus abdominis?” As the three aponeuroses approach the rectus abdominis from the lateral side, they can go anterior or posterior to the muscle. The external oblique aponeurosis goes anterior, the transverses goes posterior. The internal oblique aponeurosis splits into two layer : one passes anterior to fuse with the external oblique, the other passes posterior to merge with the transversus. The rectus muscle the thus enclosed in a sheath made of these aponeuroses.

Fig. 20 shows the rectus sheath in a transverse section, a little above the umbilicus. Note the ‘splitting’ of the internal oblique (I) aponeurosis and the relationship of the other two (E : external oblique, T : transversus). Note how all three aponeuroses of both sides converge to form the white fibrous band, the linea alba. This description is true for a large part of the rectus abdominis. In the lowest part, all three aponeuroses are in front of the rectus. At the upper end too, there are two small special regions. However, the details of these are not core knowledge for this unit. Nerves and blood vessels of the abdominal wall. You will recall that all these muscles are attached to some of ribs. Indeed, developmentally they come from thoracic segments of the body wall and spread over the abdomen. Nerves from T5 downwards supply the abdominal wall. The lowest spinal nerve to supply the abdominal wall is L1. Though not all these nerves supply all muscles, it is a good approximation. To be precise, the internal oblique and the transversus do not reach as high as the external oblique and the rectus. Conceptually, it is more important to realise that nerves T5 to T11 are in fact intercostal nerves – that is, ventral rami of thoracic spinal nerves. Since there is no rib beyond the 12th, nerve T12 is called the subcostal nerve. The nerves are accompanied by corresponding arteries and veins. The neurovascular bundles slip across the costal margin and run in the connective tissue between the internal oblique and the transversus. Compare this with the thoracic wall where the nerves and blood vessels run between the second and the third layer. This plane is called the neurovascular plane of the abdominal wall. The general plan of the abdominal wall shows that the nerves also have branches going to the skin. A branch emerging on the lateral side of the trunk supplies a large part of the skin. This is the lateral cutaneous branch. A smaller, anterior cutaneous branch, supplies the skin close to the midline. One feature of the anterior abdominal wall deserves mention at this stage. While most of the abdominal organs stay in the abdominal cavity, the testis in the male emerges out and comes to be located in the scrotum, a pouch-like extension of the abdominal wall. The passage for the testis is called the inguinal canal, which is more like three openings in the abdominal wall. This will be discussed when we study the development of the reproductive organs.

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This chapter has introduced a number concepts and a mass of factual information – both embryological and anatomical. It is important to use this text as explanatory, read it along with the pictures and distil key concepts. Some words need to be understood – this facilitates their use in your descriptions. These are summarised here. We begin with embryological concepts and words.

Parietal : This word comes from the Latin root ‘parietes’, meaning ‘wall’. In our present discussion, it refers to the body wall. The word is used in other contexts also, but we shall not worry about that now.

Visceral : The word refers to any of the internal organs. (From Latin ‘viscus’ meaning an organ. ‘Viscera’ is plural of viscus, ‘visceral’ means ‘pertaining to a viscus). Do NOT confuse this word with viscous (spelt with a ‘u’), which is an adjective, meaning thick or sticky.

The connective tissue framework of body wall is formed by the parietal layer of lateral plate mesoderm.

The epidermis of the skin is formed by the ectoderm. The skeletal muscle of the body wall is formed by myotomes in a segmental manner. Myotomes are

parts of somites, which are parts of paraxial mesoderm. The coelomic cavity is the cavity between the visceral and parietal layers of lateral plate mesoderm. Pericardial cavity (around the heart), pleural cavities (around the lungs) and peritoneal cavity (in the

abdomen) are subdivisions of the coelomic cavity. The epithelium lining these cavities is mesodermal in origin. Most viscera in the thorax and abdomen are surrounded by the coelomic cavity. They are not inside

the coelomic cavity. The coelomic cavity appears large in the diagrams shown here. In reality, it is filled only with a thin

layer of watery (‘serous’) fluid. Therefore subdivisions of the coelomic cavity are also called serous cavities. The epithelium with its very thin layer of supporting connective tissue is also called a serous membrane.

The visceral layer of a serous membrane surrounding an organ forms an integral part of the organ. Anatomy of the body wall :

Both thoracic and abdominal walls (anterolateral parts) have a common plan : skin, fascia, muscle, an inner connective tissue layer and parietal layer of a serous membrane. Check this with figs. 1 and 2.

The segmental pattern is well illustrated in the thoracic wall, with ribs and intercostal spaces. The major nerves and blood vessels come from the posterior side, reinforced by a small supply from

the anterior end. The major part of the anterior abdominal wall develops from thoracic segments. The first lumbar

segment has a small contribution at the lower end. Therefore… The nerves of the anterior abdominal wall are continuations of the lower thoracic nerves. In the thoracic wall the segmental muscles are used mainly for movements of the ribs during

respiration. In the abdominal wall the muscles form an almost complete jacket to regulate pressure within the

abdomen. The pressure also plays a great role in keeping abdominal organs in their positions. In both thoracic and abdominal walls, the main neurovascular plane is between the second and the

third layers of muscle. Check the descriptions to know the names of these muscles.

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