In anatomy, a fissure is a deep cleft or groove that divides an organ or structure into distinct sections. Fissures appear throughout the body, from the brain and lungs to the liver and skull bones, and they serve as important landmarks that help define the boundaries between lobes, regions, or compartments. The term also applies in clinical medicine, where it describes a tear in soft tissue like the lining of the anal canal.
Fissures vs. Sulci and Grooves
The classic distinction is straightforward: a fissure separates one lobe from another, while a sulcus is a shallower groove within a lobe. In the brain, for example, sulci are the wrinkled indentations between the folds (gyri) on the brain’s surface. Fissures run deeper and mark the boundaries between entire lobes. In practice, though, the terminology can blur. The lateral sulcus of the brain was historically called the Sylvian fissure, and many textbooks still use both names interchangeably. The key idea is that fissures represent the largest, deepest divisions in an organ’s architecture.
Major Fissures of the Brain
The brain has two prominent fissures that define its basic layout. The longitudinal fissure runs front to back along the midline, separating the left and right cerebral hemispheres. It’s the deep gap you’d see if you looked down at the top of an exposed brain.
The lateral fissure (Sylvian fissure) runs along each side of the brain, separating the temporal lobe below from the frontal and parietal lobes above. This fissure is notable because it forms through a unique developmental process: during fetal development, it becomes visible as early as 12 weeks of gestation, making it one of the first recognizable landmarks on the brain’s surface. Between gestational weeks 23 and 25, the frontal and temporal lobes grow outward and fold over a deeper region called the insula, gradually closing the fissure into the deep groove seen in the adult brain. No other brain fold forms this way.
Another important boundary is the central sulcus (sometimes called the Rolandic fissure), which separates the frontal lobe from the parietal lobe. It runs from the top of the brain down toward the lateral fissure and sits between two parallel ridges that handle motor control and sensory processing.
Lung Fissures
The lungs use fissures to divide into lobes, and the two sides aren’t symmetrical. The left lung has a single oblique fissure that runs diagonally, separating it into an upper (superior) lobe and a lower (inferior) lobe. The right lung has two fissures: an oblique fissure and a horizontal fissure. The oblique fissure separates the lower lobe from the middle and upper lobes, while the shorter horizontal fissure runs from the oblique fissure forward to the lung’s front edge, carving out a wedge-shaped middle lobe from the upper lobe. This gives the right lung three lobes compared to the left lung’s two.
These fissures matter in medical imaging and surgery because they define the boundaries between functionally separate sections of lung tissue. Surgeons removing a diseased lobe follow fissure lines to preserve healthy tissue.
Fissures of the Skull
Several fissures in the skull serve as passageways for nerves and blood vessels rather than simply dividing structures. The superior orbital fissure and inferior orbital fissure are narrow gaps in the bone behind each eye socket that connect the orbit to deeper spaces in the skull.
The inferior orbital fissure sits at the floor of the eye socket and allows communication between the orbit and areas like the pterygopalatine fossa (a small space deep behind the cheekbone). Nerves that provide sensation to the mid-face, along with blood vessels, travel through this gap. The superior orbital fissure connects the eye socket to the cavernous sinus, a major venous channel alongside the brain, and carries nerves that control eye movement and sensation around the forehead. These bony fissures are clinically significant because fractures or tumors near them can compress the nerves passing through, affecting vision or facial sensation.
Liver Fissures
On the liver’s surface, fissures help define the organ’s internal geography. The most visible external landmark is the falciform ligament, which marks the division between the left and right lobes on the liver’s upper (diaphragmatic) surface. On the underside, the fissure for the round ligament (a remnant of the fetal umbilical vein) and the fissure for the venous ligament (a remnant of a fetal blood vessel called the ductus venosus) serve as boundaries between liver segments.
These surface landmarks correspond to the deeper internal segmentation that surgeons use when planning liver resections. The liver is divided into eight functional segments based on its blood supply and bile drainage, and the fissures on its surface help orient those internal divisions.
Anal Fissures: A Clinical Use of the Term
Outside of normal anatomy, the word “fissure” also describes a pathological tear. An anal fissure is a longitudinal tear in the thin tissue lining the lower anal canal. It’s one of the most common reasons people visit an emergency department for rectal pain.
Acute anal fissures appear as superficial tears extending upward from the anal opening and are defined by symptoms lasting less than six weeks. Chronic fissures persist beyond six weeks and tend to be deeper, sometimes exposing the underlying sphincter muscle. Most anal fissures occur at the posterior midline (the back side of the anal canal), where blood flow is less than half that of other areas, making the tissue more vulnerable to tearing. Chronic fissures often develop visible changes at their edges, including small skin tags or raised tissue, which help distinguish them from acute tears.
Fissure Fractures in Bone
In orthopedic medicine, a fissure fracture refers to a hairline crack in a bone that doesn’t extend all the way through. The bone remains in one piece, with the break running as a thin line along its surface. This is distinct from a complete fracture, where the bone separates into two or more pieces, and from a compound fracture, where broken bone pierces the skin. Fissure fractures are often caused by repetitive stress rather than a single forceful impact, placing them in the same family as stress fractures. They can be difficult to detect on initial X-rays and sometimes require more sensitive imaging to confirm.
Why Fissures Matter as Landmarks
Across every organ system, fissures serve as reliable reference points. Radiologists use lung fissures to localize infections or tumors to a specific lobe. Neurosurgeons navigate by the longitudinal and lateral fissures when planning approaches to brain lesions. Hepatic surgeons trace surface fissures to map internal liver segments before removing tissue. The common thread is that fissures are consistent enough from person to person to function as a shared anatomical language, even though minor variations in their exact position and completeness are normal.