Lung Lobes Anatomy: Human and Animal Comparison

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

Humans have a right lung with three lobes (superior, middle, and inferior) and a left lung with two lobes (superior and inferior), separated by oblique and horizontal fissures. Dogs have two lobes on the left and four on the right, cats have three on the left and four on the right, and cattle have a cranial and a caudal lobe on each side plus an accessory lobe on the right.

Lung lobes anatomy is one of the clearest places to see how mammals share a basic respiratory blueprint while differing in the details. The lobes are the large, grossly visible divisions of each lung, and they are separated by grooves called fissures. In people, the fissures are usually well defined, which is why a chest X-ray or CT scan can be read lobe by lobe. In animals, the picture is more variable. Some species have crisp interlobar boundaries. Others have fissures that are incomplete, meaning the lobes are partly fused and the surgeon's finger cannot always pass cleanly between them.

This article walks through human lobation first, then compares the dog, cat, ruminant (cattle, sheep, and goats), and horse. It explains what fissures and the accessory lobe actually are, why the cardiac notch exists, and how this anatomy matters in clinical practice. A comparison table and a labeling list near the end make the differences easy to review.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

What a Lung Lobe Actually Is

A lobe is a major subdivision of a lung, defined by the fissures that separate it from neighboring lobes and by its own bronchial supply. Each lobe receives a primary or lobar bronchus and its own share of pulmonary vessels. That arrangement is why a surgeon can remove a single lobe (a lobectomy) without collapsing the rest of the lung.

Fissures are the clefts between lobes. They form during development, when the growing lung bud divides into bronchopulmonary segments. The intersegmental planes that persist become the interlobar fissures [1]. When those planes are not fully obliterated, the result is an incomplete fissure, an accessory fissure, or, in rare cases, a missing fissure [1][2].

Two structural features deserve definitions up front.

• Oblique fissure: the long, slanted cleft that separates the upper lung from the lower lung. Every mammal with more than one lobe per side has an oblique fissure.
• Horizontal fissure: a shorter cleft in the human right lung that separates the superior lobe from the middle lobe. It has no direct equivalent in most domestic species.

A third structure, the accessory lobe, is a small lobe that sits medially, near the hilum, and is separated from the rest of the lung by its own fissure. It is present in several species and is almost always on the right side.

Human Lung Lobes Anatomy

The human right lung has three lobes. The superior lobe occupies the apex and upper chest. The middle lobe lies anteriorly and inferiorly to the superior lobe. The inferior lobe fills the base and the posterior chest. Two fissures divide them: the oblique (major) fissure separates the superior and middle lobes above from the inferior lobe below, and the horizontal (minor) fissure separates the superior lobe from the middle lobe.

The human left lung has two lobes. The superior lobe includes a tongue-like projection called the lingula, which corresponds developmentally to the right middle lobe. The inferior lobe sits below the left oblique fissure. There is no horizontal fissure on the left in the standard description.

The cardiac notch is a concave indentation on the anteroinferior border of the left superior lobe. The heart sits in this space, which is why the left lung is slightly smaller and lighter than the right. The lingula lies just below the notch.

Human lobation is not perfectly uniform. Cadaveric and imaging studies show that fissure completeness varies widely. In one study of 81 pairs of cadaveric lungs, the left oblique fissure was complete in 81.5% of lungs, incomplete in 16.0%, and absent in 2.47%. The right oblique fissure was complete in 64.2% and incomplete in 35.8%, and it was never absent. The right horizontal fissure was the most variable: complete in only 22.2%, incomplete in 66.7%, and absent in 11.1% [2]. On average, the left oblique fissure was 97.1% complete, the right oblique fissure 91.6% complete, and the horizontal fissure only 69.4% complete [2].

These figures matter because "three lobes on the right" is a textbook norm, not a guarantee. A 2024 South African cadaveric study found incomplete oblique fissures in 25 of 48 right lungs and 30 of 48 left lungs, and incomplete horizontal fissures in 39 right lungs [3]. A Nepalese study of 50 lungs reported incomplete oblique fissures in 7 right and 14 left lungs, and a completely missing horizontal fissure in 3 specimens [4]. Accessory fissures appear in a minority of lungs, reported at roughly 16% of patients on high-resolution CT, with the accessory horizontal fissure the most common finding on the left and superior and inferior accessory fissures on the right [5].

The takeaway for human anatomy is that the classic two-and-three pattern is the reference, and normal variation is common enough that surgeons and radiologists plan around it [6][7].

Comparative Lung Lobes Anatomy Across Species

The table below summarizes lobation and fissure patterns. Read it as a general species pattern, not a rigid rule for every individual animal.

Dog Lung Lobes

The dog has two lobes on the left and four on the right. The left lung is divided into a cranial lobe and a caudal lobe. The right lung is divided into a cranial lobe, a middle lobe, a caudal lobe, and an accessory lobe [8].

The right cranial lobe in the dog is further split by a fissure into a cranial and a caudal part, which is why some texts describe the right side as having more subdivisions than a simple count of four suggests. The accessory lobe sits medially, tucked against the mediastinum near the caudal vena cava.

A key practical point is that interlobar fissures in dogs are often incomplete. The lobes are separated by grooves, but the connective tissue between them is not always a clean, full-thickness cleft. This is one reason thoracic surgery in dogs requires careful dissection rather than simple blunt separation. The incomplete fissures also mean that a lesion in one lobe can spread more easily to an adjacent lobe, because the barrier is not complete.

Cat Lung Lobes

The cat has three lobes on the left and four on the right. The left lung is divided into cranial, middle, and caudal lobes. The right lung is divided into cranial, middle, caudal, and accessory lobes. The accessory lobe is on the right, as in the dog.

Cats are sometimes described as having a lobation pattern intermediate between dogs and humans, because the left side has an extra lobe compared with the dog. As in dogs, the fissures are frequently incomplete, so the lobes are not always surgically separable by simple cleavage.

Ruminant Lung Lobes (Cattle, Sheep, and Goats)

Cattle have a distinctive pattern. Each lung has a cranial lobe and a caudal lobe, and there is an accessory lobe on the right. The right cranial lobe is divided into a cranial and a caudal part, which gives the right side an extra subdivision that the left side lacks. Sheep and goats follow a similar plan.

Ruminant lungs are also characterized by more complete separation between lobes than dogs or horses. The interlobar fissures in cattle tend to be deeper and more complete, so the lobes are more distinct. This is a useful contrast: the same general lobation plan can be expressed with very different degrees of fissure completeness across species.

The ruminant pattern is clinically relevant because cattle are prone to respiratory disease, and the cranial lobes are often the first to be affected in bronchopneumonia. The more complete fissures in cattle mean that disease can be somewhat more contained within a lobe, but the cranial lobe's position and airway anatomy still make it a common site of infection.

Horse Lung Lobes

The horse has three lobes on each side. The divisions are typically described as a cranial lobe, a caudal lobe, and a smaller third division. The accessory lobe is present on the right.

As in dogs, interlobar fissures in horses are often incomplete. The lobes are less sharply separated than in cattle. This matters for equine thoracic surgery and for interpreting imaging, because a fissure that looks complete on a radiograph may not be a full anatomical separation.

Why Fissures Are Sometimes Incomplete

Fissure completeness is not a trivial detail. It reflects how the embryonic lung bud divided and how much connective tissue persisted between segments [9]. In species with thick pleura and extensive interlobular connective tissue, the septa between lobules can be complete, while in others they are incomplete [9]. This produces functional differences in how air and fluid move through the lung and how disease spreads.

In humans, incomplete fissures are common enough that they are considered normal variants rather than anomalies [2][5]. The same is true in dogs and horses. In cattle, the fissures are more complete, so the lobes are more sharply defined.

For a veterinarian or physician, an incomplete fissure changes surgical planning. It can increase the risk of air leaks, prolong operative time, and, in human thoracic surgery, sometimes force a conversion from a minimally invasive approach to open thoracotomy [1]. The same principle applies in animals: a lobe that is partly fused to its neighbor is harder to remove cleanly.

The Cardiac Notch and the Lingula

The cardiac notch is the concave indentation on the left lung that accommodates the heart. It is most prominent in humans, where the heart occupies a large share of the left thoracic cavity. The lingula is the small tongue of lung tissue just below the notch, and it is developmentally equivalent to the right middle lobe.

In animals, the cardiac notch is less prominent or differently shaped because the heart's position and size relative to the lungs differ. Dogs and cats have a more conical thoracic cavity, and the heart sits more centrally. The left cranial lobe in the dog and cat wraps around the heart to some degree, but the notch is not described in the same way as in human anatomy.

The lingula is a human-specific term in most teaching contexts. In animals, the equivalent region is simply part of the cranial lobe on the left. This is a common point of confusion for students moving between human and veterinary anatomy: the names do not always map one-to-one.

How to Label Lung Lobes: A Review List

When labeling a lung diagram for a human, dog, cat, or ruminant, use this order.

Human right lung:

1. Superior lobe
2. Middle lobe
3. Inferior lobe
4. Oblique fissure
5. Horizontal fissure

Human left lung:

1. Superior lobe (including the lingula)
2. Inferior lobe
3. Oblique fissure
4. Cardiac notch

Dog right lung:

1. Cranial lobe
2. Middle lobe
3. Caudal lobe
4. Accessory lobe

Dog left lung:

1. Cranial lobe
2. Caudal lobe

Cat right lung:

1. Cranial lobe
2. Middle lobe
3. Caudal lobe
4. Accessory lobe

Cat left lung:

1. Cranial lobe
2. Middle lobe
3. Caudal lobe

Cattle right lung:

1. Cranial lobe (cranial part)
2. Cranial lobe (caudal part)
3. Caudal lobe
4. Accessory lobe

Cattle left lung:

1. Cranial lobe
2. Caudal lobe

This list is a labeling aid. Individual animals can vary, and imaging or dissection may reveal accessory fissures or incomplete separations not shown in a standard diagram.

Clinical Relevance, Limitations and Common Mistakes

Lung lobation matters in several practical situations.

Auscultation. When a veterinarian listens to the chest, the lobe boundaries help localize where breath sounds are coming from. In dogs, the cranial lobes are under the upper part of the chest, the middle lobe is more central, and the caudal lobes fill the lower and posterior chest. Abnormal sounds in a specific region point toward a specific lobe.

Imaging. Radiographs and CT scans are read lobe by lobe. Knowing that a dog has four right lobes and two left lobes prevents mislabeling a lesion. In humans, the high rate of incomplete horizontal fissures means a "missing" middle lobe on a scan may be a normal variant rather than pathology [2][5].

Surgery. Lobectomy requires knowing where the fissures are and whether they are complete. In dogs and horses, incomplete fissures mean the surgeon must dissect rather than simply separate. In cattle, more complete fissures make lobar boundaries clearer.

Disease spread. Complete fissures act as partial barriers to the spread of infection between lobes [6]. Incomplete fissures allow easier spread. This is one reason the same pathogen can produce different patterns of disease in different species.

Common mistakes include assuming that all animals have the human pattern, confusing the lingula with a separate lobe, and treating a fissure as complete when it is only partial. Another frequent error is counting lobes without accounting for the accessory lobe, which sits medially and is easy to miss on a quick external inspection.

The main limitation is that lobation is a general species pattern. Individual animals can have accessory fissures, missing fissures, or unusual lobe divisions. A 2021 case report described a human left lung with four lobes and three fissures, a pattern not previously reported [10]. Similar variations occur in animals. For any individual patient, a veterinarian should interpret imaging and surgical findings in context.

Frequently Asked Questions

How many lobes does a dog have in each lung?

A dog has two lobes in the left lung and four in the right lung. The right side includes the cranial, middle, caudal, and accessory lobes.

How many lobes does a cat have?

A cat has three lobes in the left lung and four in the right lung. The right side includes an accessory lobe, as in the dog.

Do cattle have an accessory lung lobe?

Yes. Cattle have an accessory lobe on the right side, along with a cranial and a caudal lobe on each side. The right cranial lobe is divided into a cranial and a caudal part.

What is the cardiac notch?

The cardiac notch is the concave indentation on the left lung that accommodates the heart. It is most prominent in humans and is associated with the lingula.

Are lung fissures always complete?

No. Fissures can be complete, incomplete, or absent. In dogs and horses, interlobar fissures are often incomplete. In cattle, the separation between lobes is more complete.

What is the lingula?

The lingula is a tongue-like projection of the human left superior lobe. It corresponds developmentally to the right middle lobe and sits below the cardiac notch.

Why do humans have three lobes on the right and two on the left?

The asymmetry reflects the space taken up by the heart on the left side. The left lung has less room, so it has two lobes and a cardiac notch, while the right lung has three.

Do horses have the same number of lobes on both sides?

Yes. Horses typically have three lobes on each side, plus an accessory lobe on the right. The interlobar fissures in horses are often incomplete.

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