“Transverse fissure” is a term that shows up in anatomy referring to at least three entirely different structures in three different organs: a deep cleft on the underside of the liver, a narrow space between the cerebral hemispheres and the cerebellum, and the horizontal division between two lobes of the right lung. The shared word “transverse” simply means the groove or gap runs roughly side to side rather than up and down. Which transverse fissure matters depends on which organ you are looking at, and each one carries its own clinical significance.
The Transverse Fissure of the Liver
In the liver, the transverse fissure is another name for the porta hepatis, the gateway on the organ’s underside where blood vessels, bile ducts, nerves, and lymphatics enter and exit. It runs roughly five centimeters across the lower surface of the right lobe, closer to the back than the front, and connects at nearly a right angle with the left sagittal fossa. That intersection divides two smaller segments of the liver: the quadrate lobe in front and the caudate lobe behind.1European Society of Radiology. Porta hepatis: Pandora’s Box, what the radiologist needs to know
If you picture the liver flipped over so you are looking at its visceral surface, the porta hepatis looks like a short horizontal slot. Through it pass the portal vein (carrying nutrient-rich blood from the gut), the hepatic artery (carrying oxygenated blood), and the common hepatic duct (draining bile toward the gallbladder and intestine). The standard arrangement places the portal vein in the middle, the hepatic artery to its left, and the bile duct somewhat to its right. A study of 30 Indian liver specimens found that 29 followed this conventional layout, with only one specimen showing an unusual pattern where the common bile duct sat in front of the portal vein rather than beside it.2Bioinformation. Porta hepatis in relation to portal vein among Indians
The relative positions of these structures shift slightly depending on how deep into the fissure you look. Near the upper margin, the left hepatic artery tends to sit clearly to the left of the portal vein. Closer to the lower margin, that artery can drift forward and to the left. Surgeons performing liver resections, transplants, or bile duct repairs need to know exactly where each structure sits for a given patient, which is why preoperative imaging of the porta hepatis is standard practice before any hepatic procedure.
Accessory Fissures and Liver Variation
The liver’s transverse fissure is a constant landmark, but livers themselves vary more than most people realize. A large cadaveric review found accessory fissures on the liver surface in over half of specimens examined, along with accessory lobes in roughly a quarter of cases and a bridging strip of liver tissue called a pons hepatis in about one in five.3Surgical and Radiologic Anatomy. A comprehensive study and extensive review of morphological variations of liver with new insights These extra grooves and lobes are usually harmless and discovered incidentally on imaging. The real issue is that an unexpected fissure can be mistaken for a fracture, abscess margin, or tumor boundary on a scan, leading to unnecessary alarm or incorrect surgical planning. A separate normal feature, the sulcus of the caudate process (sometimes called Rouviere’s sulcus), appeared in about two-thirds of the livers in that same study and serves as a useful landmark during laparoscopic cholecystectomy to help identify the bile duct safely.
The Transverse Cerebral Fissure
In the brain, the transverse fissure (formally the transverse cerebral fissure of Bichat) is the narrow gap that separates the undersurface of the cerebral hemispheres from the top of the cerebellum and brainstem. It is not a surface groove you can see by looking at the outside of the brain. Instead, it is a deep, slit-like space that communicates with the subarachnoid cisterns and contains cerebrospinal fluid, membranes, and small blood vessels.
Structurally, the fissure has a central part and two lateral wings. The central portion includes the velum interpositum, a thin membrane roofing the third ventricle, plus the quadrigeminal plate cistern behind it. At its front end, this central channel closes near the foramina of Monro (the passages linking the lateral ventricles to the third ventricle). At its back end, it opens into the quadrigeminal cistern, a small cerebrospinal fluid pool that sits behind the midbrain.4The Journal of Maternal-Fetal & Neonatal Medicine. Imaging of the transverse cerebral fissure at three-tesla MR fetal autopsy The two lateral wings angle forward and downward toward the inner surfaces of the temporal lobes. All of this makes the transverse cerebral fissure less a single crack and more a three-dimensional corridor with branching arms.
Because this fissure is essentially a virtual space that is only visible when something fills it, it can be surprisingly hard to study in living patients. In normal MRI scans, the gap is so thin it may not register clearly. Researchers at a fetal autopsy center managed to define its borders in fetuses at about 20 weeks of gestation by using a powerful three-tesla MRI scanner. Postmortem blood breakdown products (hemosiderin) had settled into the fissure’s boundaries, staining them dark on T2-weighted images and effectively outlining the entire space.5The Journal of Maternal-Fetal & Neonatal Medicine. Imaging of the transverse cerebral fissure at three-tesla MR fetal autopsy That accidental contrast helped confirm the fissure’s shape during early development and illustrates how tricky it is to visualize a structure that barely exists as a physical gap in a healthy brain.
Why It Carries Bichat’s Name
The transverse cerebral fissure is often called Bichat’s fissure after Marie-François Xavier Bichat, a French anatomist who worked at the turn of the nineteenth century. Bichat died at just 30 years old but left an outsized mark on anatomy and pathology. His name is attached to at least half a dozen structures: the pterygopalatine fossa (Bichat’s fossa), the buccal fat pad (Bichat’s protuberance), a ligament in the sacroiliac joint, the innermost coat of blood vessels, and the transverse cerebral fissure itself.6PubMed. Marie-François Xavier Bichat (1771-1802) and his contributions to the foundations of pathological anatomy and modern medicine The tradition of naming structures after their discoverers has largely fallen out of favor in modern anatomy, where descriptive terms are preferred. But Bichat’s fissure remains in common use, especially in neurosurgical and neuroradiological literature, so the term still comes up when surgeons discuss approaches to tumors near the pineal gland or the back of the third ventricle.
The Horizontal Fissure of the Right Lung
The third major “transverse fissure” in the body is in the right lung. The right lung has three lobes (upper, middle, and lower), separated by two fissures. The oblique fissure runs diagonally from back to front, dividing the lower lobe from the rest. The horizontal fissure, sometimes called the transverse fissure, runs roughly level from the oblique fissure forward to the anterior lung border, separating the upper and middle lobes. The left lung, which has only two lobes, does not normally have a horizontal fissure at all.
In textbook diagrams the horizontal fissure is drawn as a clean, complete line. Reality is messier. A cadaveric study of 81 right lungs found the horizontal fissure was complete in only about a fifth of cases. In roughly two-thirds it was incomplete, meaning the fissure started but did not extend all the way across the lung surface. In about one in nine lungs, it was absent altogether.7PubMed. Are textbook lungs really normal? A cadaveric study on the anatomical and clinical importance of variations in the major lung fissures, and the incomplete right horizontal fissure On average, the fissure was only about 70 percent complete, with the gap most often found on the inner front portion of the lung. Another cadaveric series reported the horizontal fissure absent in about 7 percent of right lungs and incomplete in 39 percent.8Translational Research in Anatomy. Anatomical variations in lung fissures leading to supernumerary lobes in the lungs The exact numbers vary from study to study, but the overall picture is clear: a fully complete horizontal fissure is the minority finding, not the norm.
Why Incomplete Lung Fissures Matter
When the horizontal fissure is incomplete, the upper and middle lobes of the right lung are partially fused, connected by a bridge of lung tissue where the fissure should have been. This has practical consequences. In pneumonia, for instance, an infection that starts in one lobe can spread into the adjacent lobe through that tissue bridge more easily than it could if a complete fissure formed a barrier. Radiologists sometimes see a finding called the “incomplete fissure sign,” where fluid or consolidation seems to cross between lobes, and an incomplete fissure is one explanation for that pattern.9PubMed. Radiographic and CT appearances of the major fissures
Air can also drift from one lobe to another through an incomplete fissure, a phenomenon called collateral air drift. This matters during procedures like endobronchial valve placement, a treatment for severe emphysema in which a one-way valve is placed in a bronchus to deflate a hyperinflated lobe. If the fissure between the target lobe and its neighbor is incomplete, air leaks back in through the tissue bridge and the lobe fails to collapse. For this reason, CT scans or specialized assessments of fissure completeness are a routine part of planning for those procedures.
Beyond the standard fissures, some people have accessory fissures that create extra lung divisions. A CT-based study of nearly 400 patients found accessory fissures in about 7 percent of cases. The most common accessory fissure was the azygos fissure, present in close to 4 percent, followed by the superior accessory fissure and inferior accessory fissure at about 1 to 2 percent each.10PubMed Central. Anatomical Variations in Pulmonary Fissures on Computed Tomography (CT) Like liver accessory fissures, these lung variants are usually harmless but can confuse interpretation of imaging or complicate surgery if the surgeon does not realize an extra lobe is present.
Comparing the Three Transverse Fissures
Even though the name is the same, these three structures differ in almost every way that matters. The hepatic transverse fissure is a passageway for blood vessels and ducts, a transit zone where structures enter and leave the liver. The cerebral transverse fissure is a cerebrospinal fluid-filled cleft that serves mainly as a boundary and contains membrane layers and small veins. The pulmonary horizontal fissure is a surface groove created during fetal development as the lung buds divide into lobes, and it functions as a partial physical barrier between lobes.
Their clinical relevance differs, too. In the liver, the porta hepatis is a routine surgical landmark. In the brain, the transverse cerebral fissure is most relevant during deep neurosurgical approaches where the surgeon navigates through or near it to reach structures like the pineal gland or the roof of the third ventricle. In the lungs, the fissure’s main clinical role is in imaging interpretation and procedure planning, particularly for lobar collapse therapies in emphysema.
Variability also plays out differently. The liver’s porta hepatis is always present and always in the same general location, though the arrangement of vessels within it shifts from person to person. The cerebral transverse fissure is similarly consistent in its existence but difficult to visualize. The lung’s horizontal fissure, by contrast, is frequently incomplete or absent, making it the least reliable of the three as a landmark.
The Transverse Palmar Crease
Outside the internal organs, the word “transverse” also appears in a surface feature most people can see on their own hands. The transverse palmar crease (sometimes still called the simian crease, though that term is falling out of use) is a single crease that runs straight across the palm rather than the more common pattern of two or three curved lines. In most hands, the major palm creases angle in different directions, reflecting the way the skin folds when the hand grips and flexes. A study of over 800 palms among medical and dental students in Addis Ababa found aberrant crease patterns in about 14 percent, with the simian crease being the most common variant, followed by other unusual patterns.11PubMed Central. Prevalence of the Different Types of Palmar Creases Among Medical and Dental Students in Addis Ababa, Ethiopia
Researchers have developed quantitative methods to analyze how transverse a given palm crease is, using coordinate grids mapped onto palm prints to calculate a “total degree of transversality” for each crease.12PubMed Central. Improved analysis of palm creases This kind of measurement matters because an unusually straight, transverse crease has been associated in clinical genetics with certain chromosomal conditions, though it also occurs in a meaningful percentage of people with no medical issues at all. The connection to the term “transverse fissure” is loose but etymologically parallel: in both cases, “transverse” just marks a horizontal or side-to-side orientation rather than a vertical one.
When “Fissure” Does Not Mean “Crack”
One common source of confusion for people encountering this term for the first time is the word “fissure” itself. In everyday language a fissure sounds like a fracture or a crack, something that formed by breaking. In anatomy, fissures are normal developmental features, grooves or clefts that form as organs grow and fold during embryonic life. The lung fissures form as the bronchial tree branches and lobes separate. The liver’s porta hepatis forms where the developing biliary and vascular structures enter the organ. The cerebral transverse fissure emerges as the telencephalon (the part of the brain that becomes the cerebral hemispheres) grows backward over the diencephalon and cerebellum during fetal development.
None of these fissures represent damage. A fissure on a chest X-ray does not mean the lung is torn. A fissure at the porta hepatis does not mean the liver is splitting. This is worth keeping in mind when reading a radiology report that mentions a fissure, since the word can sound alarming to someone who has not encountered it in its anatomical sense. In virtually all contexts, a transverse fissure noted on imaging is a normal structure whose completeness, visibility, or relationship to nearby anatomy the radiologist is describing for the surgeon’s benefit.