Cephalic Trunk: Brachiocephalic Artery Anatomy

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

The cephalic trunk, also called the brachiocephalic trunk or innominate artery, is the first major vessel to branch from the aortic arch. In humans it is a short, wide artery that splits into the right common carotid and right subclavian arteries, while in dogs and cats it is a larger trunk that supplies both common carotid arteries and the right subclavian artery, with the left subclavian arising separately from the arch.

That single sentence hides a surprising amount of comparative anatomy. The number of vessels leaving the aortic arch, their order, and which structures they feed differ across mammals, and those differences matter to surgeons, anesthesiologists, and anyone interpreting thoracic imaging. This article walks through the embryology, the species-specific branching patterns, and the clinical relevance of the cephalic trunk.

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

What the Cephalic Trunk Is

The cephalic trunk is a large elastic artery that arises from the convex (outer) curvature of the aortic arch. The word "cephalic" refers to the head, and "brachiocephalic" combines brachium (arm or forelimb) with cephalic (head), because the vessel supplies both regions. "Innominate" is an older synonym meaning "unnamed," used because the artery was historically described as having no distinct name of its own before it divided.

In humans, the brachiocephalic trunk is the first of three branches from the aortic arch. The other two are the left common carotid artery and the left subclavian artery. The trunk itself is short, typically 3 to 5 cm in adults, and divides behind the right sternoclavicular joint into the right common carotid artery and the right subclavian artery. This arrangement is asymmetric: the right side gets a common trunk while the left side has two separate vessels. The asymmetry reflects the embryologic remodeling of the aortic sac and the paired aortic arches.

In dogs and cats, the cephalic trunk is proportionally larger and longer relative to body size. It gives off both common carotid arteries and the right subclavian artery. The left subclavian artery arises separately from the aortic arch, just distal to the cephalic trunk. This is the classic carnivore pattern and is the arrangement most veterinary students are expected to recognize on dissection.

Embryology in Plain Language

To understand why species differ, it helps to know how the aortic arch system forms. Early in development, the embryo has a series of paired aortic arches (pharyngeal arch arteries) that connect the ventral aorta (the future ascending aorta and aortic sac) to the paired dorsal aortae. These arches are numbered 1 through 6, though not all persist.

In mammals, the third, fourth, and sixth arches contribute to the definitive great vessels. The fourth arch derivatives are the ones most relevant here:

• On the left, the fourth arch becomes the definitive aortic arch.
• On the right, the fourth arch becomes the proximal part of the right subclavian artery.
• The right common carotid artery derives from the right third arch and the right horn of the aortic sac.
• The left common carotid artery derives from the left third arch and the left horn of the aortic sac.

The brachiocephalic trunk forms from the right horn of the aortic sac plus the proximal right fourth arch. In humans, this produces a single trunk that divides into the right common carotid and right subclavian. In dogs and cats, the same embryologic territory expands to include the origin of the left common carotid as well, which is why the carnivore cephalic trunk gives off both carotids.

This is a simplification, but it explains the core concept: the cephalic trunk is not a fixed structure across mammals. It is a segment of the aortic sac and fourth arch that gets remodeled differently in each lineage.

Species Comparison of the Aortic Arch Branches

The table below summarizes the branching pattern of the aortic arch in the species most commonly encountered in veterinary and comparative anatomy. The "adult vessels" column lists the branches in order from proximal to distal along the arch.

The pattern in dogs and cats is well documented. A study of 36 raccoon dogs (Nyctereutes procyonoides), a canid species, found that in 88.6% of cases the brachiocephalic trunk branched off the left common carotid artery and terminated in the right common carotid and right subclavian arteries [1]. In the remaining 11.4%, the brachiocephalic trunk formed a bicarotid trunk [1]. This variation is worth noting because it shows that even within a single species, the exact branching pattern can differ.

A study of Geoffroy's cat (Leopardus geoffroyi) found that in all 15 specimens examined, the aorta first gave rise to the brachiocephalic trunk, followed by the left subclavian artery [2]. In 13 of 15 animals (86.7%), a bicarotid trunk originated from the brachiocephalic trunk, with a length ranging from 0.4 to 2.5 cm (mean 1.3 cm) [2]. In the remaining two individuals (13.3%), the bicarotid trunk was absent, and the left common carotid, right common carotid, and right subclavian arteries arose directly from the brachiocephalic trunk [2].

These findings confirm that the carnivore cephalic trunk is a major vessel supplying both sides of the head and neck, plus the right forelimb. The left forelimb is supplied by the left subclavian artery, which arises independently from the aortic arch.

The Cephalic Trunk in Humans: A Brief Comparative Note

Human anatomy is often taught first, so it is useful to clarify the human pattern. In humans, the brachiocephalic trunk is the first branch of the aortic arch. It ascends superiorly and slightly to the right, then divides into the right common carotid artery and the right subclavian artery. The left common carotid and left subclavian arteries arise separately from the arch.

This arrangement is asymmetric and reflects the embryologic remodeling described earlier. The human pattern is the reference point for most medical anatomy, but it is not the pattern seen in dogs, cats, horses, or ruminants. Veterinary students must learn the carnivore pattern separately.

Variations in the human aortic arch branching pattern are common. A study of 1,000 patients using computed tomography angiography (CTA) found a normal branching pattern (Type 1) in 853 cases, while variations were found in 147 cases [3]. The most common variation was a common trunk of the brachiocephalic trunk and left common carotid artery, observed in 8.3% of cases [3]. Another study of 400 patients found typical branching in 75.5% of cases, with brachiocephalic trunk variations being the second most common at 19.5% [4]. A study of 2,923 Japanese patients found the classic pattern in 86.9% of cases, with variations in 13.1% [5].

These numbers matter because they show that even in humans, the "textbook" pattern is not universal. In veterinary medicine, the same principle applies: the carnivore pattern is the norm, but variations exist.

Branches of the Cephalic Trunk in Dogs and Cats

In dogs and cats, the cephalic trunk typically gives off three main vessels:

1. Right common carotid artery: Supplies the right side of the head and neck.
2. Left common carotid artery: Supplies the left side of the head and neck. In many animals, the left and right common carotid arteries share a short common origin called the bicarotid trunk.
3. Right subclavian artery: Supplies the right forelimb, thoracic wall, and neck.

The bicarotid trunk is a short segment that arises from the cephalic trunk and then splits into the left and right common carotid arteries. In Geoffroy's cat, the bicarotid trunk was present in 86.7% of specimens and ranged from 0.4 to 2.5 cm in length [2]. In raccoon dogs, the bicarotid trunk was present in 11.4% of cases [1]. The difference in prevalence between species suggests that the bicarotid trunk is not a fixed feature but a common variant.

The right subclavian artery continues as the axillary artery after crossing the first rib. Along its course, it gives off several branches, including the vertebral artery, costocervical trunk, superficial cervical artery, and internal thoracic artery. The order of these branches varies. In raccoon dogs, the vertebral artery branched first in 98.6% of cases, the costocervical trunk second in 81.9%, the superficial cervical artery third in 62.5%, and the internal thoracic artery fourth in 52.8% [1]. This level of detail is mainly of anatomical interest, but it matters during surgical approaches to the thoracic inlet.

The Left Subclavian Artery: The Other Side

The left subclavian artery arises directly from the aortic arch in dogs and cats, just distal to the cephalic trunk. It supplies the left forelimb and parts of the thoracic wall and neck. Because it arises separately, the left forelimb has a different vascular supply route than the right forelimb.

This asymmetry is clinically relevant. During procedures that involve the aortic arch or the thoracic inlet, the surgeon must know which vessel is which. The cephalic trunk is typically the first and largest branch, and the left subclavian is the second. In horses and ruminants, the pattern is similar: the cephalic trunk gives off the bicarotid trunk and the right subclavian, while the left subclavian arises separately.

Species Differences in Horses and Ruminants

In horses and ruminants, the cephalic trunk is a short, wide vessel that gives off the bicarotid trunk and the right subclavian artery. The bicarotid trunk then divides into the left and right common carotid arteries. The left subclavian artery arises separately from the aortic arch.

This arrangement is similar to dogs and cats, but there are subtle differences. In horses, the cephalic trunk is relatively short, and the bicarotid trunk is well developed. In ruminants, the cephalic trunk may also give off the right vertebral artery or other branches, depending on the species. The exact pattern varies, but the general principle holds: the cephalic trunk supplies both carotids and the right subclavian, while the left subclavian arises independently.

For veterinary students, the key takeaway is that the carnivore pattern is not unique. The same basic plan, a cephalic trunk supplying both sides of the head and the right forelimb, is seen in horses, ruminants, and pigs. The human pattern, with a smaller cephalic trunk supplying only the right side, is the outlier.

Clinical Relevance, Limitations and Common Mistakes

The cephalic trunk is not just an anatomy exam topic. It appears in clinical practice in several ways.

Thoracic surgery and interventional procedures. The cephalic trunk is a key landmark during aortic arch surgery. In humans, endovascular repair of the aortic arch often involves placing branch devices into the brachiocephalic trunk, left common carotid artery, and left subclavian artery. A study of 75 patients undergoing total endovascular aortic arch repair found that smaller brachiocephalic trunk branch diameter was significantly correlated with severe endograft rotation (P < .001) [6]. This means that the size of the cephalic trunk can affect the technical success of the procedure.

Dissection and aneurysm. Aortic dissection can extend into the cephalic trunk. In a study of 71 patients evaluated for endovascular repair after hemiarch replacement for acute type A aortic dissection, extension of dissection into the brachiocephalic trunk was one reason patients were deemed unsuitable for branched endograft repair [7]. This highlights the clinical importance of the cephalic trunk in aortic disease.

Imaging and anatomical variations. The cephalic trunk can be absent or have an unusual origin. A case report described a patient with an absent brachiocephalic trunk and five distinct major vessels arising directly from the aortic arch [8]. Another case report described a left-sided brachiocephalic trunk in a patient with situs inversus totalis, discovered incidentally on carotid duplex ultrasound [9]. These variations are rare but can complicate surgical planning and imaging interpretation.

Common mistakes in anatomy. The most common mistake is confusing the human pattern with the carnivore pattern. In humans, the brachiocephalic trunk gives off only the right common carotid and right subclavian. In dogs and cats, it gives off both common carotid arteries and the right subclavian. Students who memorize the human pattern may incorrectly assume that the left common carotid arises directly from the aortic arch in dogs and cats. It does not. The left common carotid arises from the cephalic trunk, often via a bicarotid trunk.

Another common mistake is assuming that the left subclavian artery is a branch of the cephalic trunk. In dogs, cats, horses, and ruminants, the left subclavian arises separately from the aortic arch. This is a key difference from the human pattern, where the left subclavian also arises separately but the right subclavian arises from the cephalic trunk.

Limitations. The exact branching pattern of the cephalic trunk varies between individuals and species. The percentages cited in this article come from specific studies and may not apply to every animal. For any clinical decision, individual anatomy must be confirmed with imaging or direct observation. This article is educational and is not a substitute for veterinary diagnosis or treatment.

Frequently Asked Questions

What is the cephalic trunk?

The cephalic trunk, also called the brachiocephalic trunk or innominate artery, is the first major vessel to branch from the aortic arch. It supplies blood to the head, neck, and forelimbs.

Is the cephalic trunk the same as the brachiocephalic artery?

Yes. Cephalic trunk, brachiocephalic trunk, and innominate artery are synonyms for the same vessel. The term "innominate" is older and less commonly used in veterinary anatomy.

What does the cephalic trunk supply in dogs?

In dogs, the cephalic trunk gives off both common carotid arteries and the right subclavian artery. The common carotid arteries supply the head and neck, and the right subclavian artery supplies the right forelimb and parts of the thoracic wall.

Does the cephalic trunk give off the left subclavian artery?

No. In dogs, cats, horses, and ruminants, the left subclavian artery arises separately from the aortic arch, just distal to the cephalic trunk.

How does the human brachiocephalic trunk differ from the dog's?

In humans, the brachiocephalic trunk gives off only the right common carotid and right subclavian arteries. In dogs, it gives off both common carotid arteries and the right subclavian artery.

What is a bicarotid trunk?

A bicarotid trunk is a short common segment that arises from the cephalic trunk and then splits into the left and right common carotid arteries. It is present in most cats and in some dogs.

Can the cephalic trunk be absent?

Yes, but it is rare. A case report described a patient with an absent brachiocephalic trunk and five major vessels arising directly from the aortic arch [8]. This is an unusual variation.

Why does the cephalic trunk matter in surgery?

The cephalic trunk is a key landmark in aortic arch surgery and endovascular repair. Its size and branching pattern can affect the feasibility and success of these procedures.

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Sources

1. Morphology of the aortic arch branching pattern in raccoon dogs (Nyctereutes procyonoides, Gray, 1834).
2. Collateral arteries of the aortic arch of the Geoffroy's cat (Leopardus geoffroyi).
3. Evaluation of aortic arch branching pattern variations with computed tomography angiography images.
4. Identifying Aortic Arch Branching Variations Using Advanced Imaging Techniques.
5. Anatomical variations in the branches of the aortic arch in Japanese patients.
6. Incidence of endograft rotation during endovascular aortic arch repair.
7. Feasibility of endovascular aortic arch repair after hemiarch replacement for acute type A aortic dissection.
8. A rare variation of five major vessels arising from the aortic arch with an absence of brachiocephalic trunk.
9. Incidental Detection of Situs Inversus Totalis Following Identification of an Unexpected Left-Sided Brachiocephalic Trunk on Carotid Duplex Ultrasound: A Case Report.