In textbooks and so forth, you might read about how the abdomen can be divided up into 9 regions, like a noughts and crosses board. The two vertical lines subdividing the abdomen are in line with the middle of the clavicle, and are called "midclavicular lines." There are two horizontal lines: one is in line with the bottom of the ribs (subcostal line) and the other is in line with the tubercles of the pelvic iliac bone (intertubercular line).
Here are the 9 regions of the abdomen, in a table, because tables are cool.
| Right hypochondrium | Epigastric | Left hypochondrium |
| Right lumbar | Umbilical | Left lumbar |
| Right iliac/inguinal | Hypogastric | Left iliac/inguinal |
Describe the layered structure of the abdominal wall and its function
The abdominal wall has several important functions. Aside from protecting the abdominal organs, the muscles of the abdominal wall can also contract and relax to change the pressure within the abdominal cavity. This, in turn, affects processes such as breathing, defecation and so on.
The abdominal wall is made up of many layers. The innermost layer is the parietal peritoneum, which surrounds the peritoneal cavity, as mentioned in my post about coelomic cavities. Surrounding this is a layer of extraperitoneal fat and then the transversalis fascia. Next up there are the three main muscles of the abdomen: the transversus abdominus (which has fibres running transversely), internal oblique (which has fibres running medially and superior- i.e. from bottom up) and external oblique (which has fibres running medially and inferior). The aponeuroses (tendinous parts) of these three muscles combine to form the rectus sheath, which surrounds the rectus abdominus which lies at the front of the abdomen.
A quick note on the rectus sheath: it has two layers, anterior and posterior. The anterior layer is made up of the aponeurosis from the external oblique as well as part of the aponeurosis from the internal oblique. The posterior layer is made up of the rest of the aponeurosis from the internal oblique as well as the aponeurosis of the transversus abdominus. Towards the bottom, however, all three layers pass in front of the rectus abdominus. The point at which this happens is the "arcuate line." Because of this, the abdominal wall is weaker below the arcuate line.
Another quick note: the medial edge of the rectus sheath (i.e. the part of the sheath between the two "halves" of the rectus abdominus) is called the linea alba, and the lateral edge is called the linea semilunaris.
Anyway, back to the layers. Outside of the muscles there is another fascia, called the superficial layer. It has a fatty layer, as well as a membranous layer. This membranous layer is called Scarpa's fascia and continues down into the scrotum or labia to become Colle's fascia. Finally, outside this fascia there is skin.
Understand how and why the wall structure differs between the sexes
Obviously, one of the main anatomical differences between the sexes is that males have penises while women have vaginas. Males also have testes, and when they descend, they drag some of the body wall into the scrotum with them. Some of the peritoneal cavity gets dragged along too, so each testis has a mini cavity called the tunica vaginalis.
The scrotum has pretty much all the same layers as the rest of the abdominal wall, just renamed or with some slight differences. The skin of the scrotum also has a layer of muscle called dartos muscle, which contracts the scrotum when it gets cold. Scarpa's fascia is now called Colle's fascia, as mentioned above. The external oblique is now the external spermatic fascia, and the transversalis fascia now the internal spermatic fascia. The main difference is what lies in between them: the internal oblique and transverse abdominus combine to develop the cremaster muscle and fascia. (That's the muscle that draws the testes closer to the body when it gets cold.)
Understand how sex determines risk of hernia
To talk about this, first I need to describe what a hernia is. A hernia is a weakness in the body wall that allows stuff to poke through when the abdominal pressure is increased.
The most common type of hernia is the inguinal hernia. The inguinal canal is a canal that passes through the aponeuroses of the three main abdominal muscles. It runs obliquely (i.e. diagonally), from the deep inguinal ring above the halfway point of the inguinal ligament, to the superficial inguinal ring at the medial end of the inguinal ligament. The spermatic cord (in males) or round ligament/ ligamentum teres (in females) pass through here, but in a hernia other things can pass through too.
There are two main types of inguinal hernia. The first, indirect inguinal hernia, is usually congenital. It occurs when the processus vaginalis (the peritoneum that descends into the scrotum) doesn't fuse shut, creating what is known as a "patent processus vaginalis." Abdominal organs can then herniate through the spermatic cord (males) or round ligament (females). The second type of inguinal hernia, direct inguinal hernia, is usually acquired as a result of weak musculature. This hernia occurs through the superficial inguinal ring.
As males have more stuff going through the inguinal canal (spermatic cord vs. round ligament), and they also have the whole issue of the processus vaginalis descending into the scrotum which may or may not close, they are much more at risk of getting inguinal hernias than females.
Females are, however, more at risk of femoral hernias. This is because their femoral canals (where femoral arteries and veins pass through) are much larger than males.
Hernias are bad because they can result in blood vessels or organs getting squished. There may be some ways of preventing them though. Aside from indirect inguinal hernias which are usually congenital, most hernias are acquired through strenuous activities such as childbearing, heavy coughing, weightlifting etc. This is why weightlifters often wear a belt to support their abdominal wall.
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