The appendix is a small, narrow tube of tissue that looks something like a short, fleshy worm dangling from the large intestine. It’s typically pinkish-gray on the outside, with a smooth, glistening surface when healthy. In size, it usually measures about 5 to 10 centimeters long (roughly 2 to 4 inches) and less than 6 millimeters in diameter, making it roughly the width of a pencil.
Shape and Surface Appearance
The appendix is a hollow, dead-end tube, closed at one end and open at the other where it connects to the large intestine. Its medical name, “vermiform appendix,” literally translates to “worm-shaped,” which is a pretty accurate description. It tapers slightly toward its closed tip and has a soft, flexible texture. The outer surface is covered in a thin, moist membrane called the serosa, the same smooth lining that covers most of your abdominal organs and gives them that slightly shiny look.
If you were to slice the appendix in cross-section, you’d see the same four-layered structure found throughout the rest of the gut: an inner lining, a middle layer packed with immune tissue, a muscular wall, and that smooth outer coating. The opening where the appendix meets the large intestine sits about 2.5 centimeters below the valve connecting the small and large intestines, and it’s sometimes partially covered by a small flap of tissue.
Where It Sits in the Body
The appendix hangs off the cecum, which is the very first section of the large intestine, located in the lower right side of your abdomen. Picture the spot roughly between your belly button and your right hip bone. The cecum acts as a pouch where the small intestine empties into the large intestine, and the appendix projects off this pouch like a small finger.
What many people don’t realize is that the appendix doesn’t always point in the same direction. A study of 377 cadavers found that the most common position is tucked behind the cecum (about 44% of people), but in roughly a quarter of people it hangs below the cecum, and in about 14% it sits behind loops of the small intestine. Around 9% of people have a pelvic appendix, meaning it droops down toward the pelvis. These variations are completely normal but can make appendicitis harder to diagnose, since the pain may show up in slightly different spots depending on where your appendix happens to sit.
What It Looks Like on Medical Imaging
On an ultrasound or CT scan, a healthy appendix appears as a thin, tube-like structure with a diameter under 6 millimeters. It can be surprisingly hard to spot when it’s healthy because it blends in with surrounding tissue and is often partially hidden behind the cecum.
When the appendix becomes inflamed (appendicitis), the difference on imaging is dramatic. The tube swells beyond 6 millimeters in diameter, the walls become visibly thicker and brighter after contrast dye is given, and the surrounding fat and tissue look hazy or streaky from inflammation. On ultrasound, a swollen appendix appears as a round, target-like structure in cross-section. This size threshold of 6 millimeters is one of the key measurements radiologists use to distinguish a normal appendix from one that needs surgical attention.
What an Inflamed Appendix Looks Like
A healthy appendix is pale pink and pliable. An inflamed one changes rapidly. In the early stages, it becomes red and swollen, similar to how a cut on your skin looks when it’s infected. As inflammation progresses, the appendix can swell to several times its normal diameter, turning a deeper red or even purplish color as blood flow is compromised. The surface loses its smooth, shiny appearance and may become coated with a yellowish film of pus.
In severe cases, the wall of the appendix can become so weakened that it develops dark, gangrenous patches where the tissue has started to die. If it ruptures, the tip or side of the appendix will have a visible hole or tear, often with pus and infected fluid leaking out. Surgeons sometimes describe a badly infected appendix as looking like an overripe, discolored sausage compared to the slim, healthy tube it once was.
What the Appendix Actually Does
For decades, the appendix was considered a useless leftover from evolution. That view has shifted. The appendix is now thought to serve as a reservoir for beneficial gut bacteria. After an illness like food poisoning or a severe intestinal infection that wipes out your normal gut flora, the bacteria sheltered inside the appendix can repopulate your intestines and help restore healthy digestion.
The appendix is also rich in immune tissue, particularly in childhood and young adulthood. Its inner walls contain dense clusters of lymphoid tissue, similar to what you’d find in your tonsils, which help the body monitor intestinal contents and mount immune responses. That said, people who have their appendix removed live perfectly normal, healthy lives, so while it plays a supporting role, it’s not essential.