What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Vertebral Column: Vertebra Anatomy and Regions Explained

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-19

The vertebral column is the segmented bony axis of the body, built from a series of individual bones called vertebrae that run from the skull to the tail. Each vertebra is a repeating unit with a body, an arch, and a set of processes, and the column is divided into five regions: cervical, thoracic, lumbar, sacral, and caudal (or coccygeal).

This matters because the vertebral column does three jobs at once. It supports the trunk and carries the weight of the body, it protects the spinal cord inside a bony canal, and it provides the levers and attachment points that muscles use to move the neck, back, and tail. For anyone working with animals, the number and shape of vertebrae in each region also carry real information: they help you read a radiograph, understand why a dog's back bends the way it does, and recognize which part of the spine a problem is likely to involve.

What the Vertebral Column Is

The vertebral column is a stack of bones, the vertebrae, linked by joints and soft tissues into a single flexible rod. It is part of the axial skeleton, the division of the skeleton that includes the skull, ribs, and sternum. The appendicular skeleton, by contrast, is the limbs and their girdles.

A vertebra is not a solid block. It is a complex shape with a central body, a ring of bone behind it that forms a canal, and several projections that muscles and ribs attach to. The canal holds the spinal cord, and the projections give the back its mechanical leverage. Because each vertebra repeats this basic plan, anatomists can describe almost any vertebra using the same vocabulary.

The number of vertebrae varies enormously across vertebrates. Snakes can have hundreds of precloacal vertebrae, and their column is regionalized into cervical, anterior thoracic, posterior thoracic, and lumbar domains based on changes in vertebral shape [1]. Mammals are far more constrained. The number and identity of vertebrae in a given species, called its axial formula or vertebral formula, is often remarkably stable [2].

The Five Regions of the Vertebral Column

The column is divided into regions based on the shape of the vertebrae, the presence of ribs, and the position along the body axis. Each region has a standard abbreviation: C for cervical, T for thoracic, L for lumbar, S for sacral, and Cd for caudal.

Cervical Region

The cervical region is the neck. Its most striking feature is how constant the count is across mammals. Almost all mammals have seven cervical vertebrae, abbreviated C7. This near-constant count is one of the most reliable rules in comparative anatomy. The ferret study cited here found that the number of cervical vertebrae was constant in all 172 animals examined, even though the thoracic, lumbar, and sacral counts varied [3].

The first two cervical vertebrae are highly modified and have their own names. The atlas (C1) is a ring-shaped vertebra that supports the skull and allows the head to nod. The axis (C2) has a peg-like projection called the dens, or odontoid process, that fits into the atlas and allows the head to rotate. These two vertebrae are so different from the rest of the column that they are usually described separately. The atlantoaxial segment is unique for its distinctive anatomy, and surgeons who operate there must know its variations well [4].

Thoracic Region

The thoracic region is the chest. Thoracic vertebrae are defined by their connection to ribs. Each thoracic vertebra has facets, or small flat articular surfaces, where a rib attaches. The number of thoracic vertebrae therefore matches the number of rib-bearing segments.

In the ferret, thoracic counts of 14 or 15 were recorded, and the formula C7/Th14/L6/S3 was the most common, appearing in just over half of the animals [3]. This shows that even within one species, the thoracic and lumbar counts can shift.

Lumbar Region

The lumbar region is the lower back. Lumbar vertebrae are typically the largest and most robust of the presacral vertebrae, and they lack rib attachments. Their transverse processes are prominent, a point covered in more detail below.

The modal number of lumbar vertebrae in modern humans is five [5]. In dogs, the standard count is seven. Lumbar vertebrae are the main weight-bearing and force-transferring segments of the lower spine in quadruped mammals.

Sacral Region

The sacral region is where the spine meets the pelvis. In most mammals, the sacral vertebrae are fused into a single structure called the sacrum. The number of sacral vertebrae varies by species and even within a species. In the ferret, both S3 and S4 formulas were recorded [3].

Caudal Region

The caudal region, also called the coccygeal region in humans and some other species, is the tail. This is the most variable region of all. The number of caudal vertebrae differs widely between species and even between individuals of the same species, which is why it is usually written as "variable" in a vertebral formula.

Parts of a Typical Vertebra

A typical vertebra has a consistent set of parts. Once you learn these, you can describe any vertebra in any region. The names come from Latin and Greek roots, and each one describes a position or shape.

The Body (Centrum)

The body, also called the centrum, is the thick, disc-shaped block at the front (ventral side) of the vertebra. It is the main weight-bearing part. Bodies are stacked one on top of another, separated by intervertebral discs, and they form the front wall of the spinal canal. In the fetus, the body develops from its own ossification center, which grows in a predictable pattern. A study of the fetal twelfth thoracic vertebra found that the body's transverse and sagittal diameters, cross-sectional area, and volume all increased linearly with gestational age [6].

The Vertebral Arch

The vertebral arch is the ring of bone behind the body that encloses the spinal canal. It is made of two pedicles and two laminae.

The Pedicles

The pedicles are the two short, thick pillars that connect the arch to the body. They form the sides of the vertebral canal. In surgical anatomy, the pedicle is a key landmark because screws are often placed through it into the body. A study of lumbar pedicle screw insertion measured the distance from the lateral wall of the pedicle to the inner wall of the pedicle and to the posterior wall of the vertebral body, finding mean distances that varied by level from roughly 16 to 20 mm depending on the measurement point [7].

The Laminae

The laminae are the two flat plates that complete the arch behind the pedicles. They meet in the midline at the back of the vertebra. Together, the pedicles and laminae form a bony ring around the spinal cord.

The Spinous Process

The spinous process is the single projection that points backward (dorsally) from the junction of the two laminae. It is the part of a vertebra you can feel through the skin along the spine of a person or a dog. Spinous processes are tallest in the thoracic region, where they overlap like roof tiles and serve as attachment points for the muscles and ligaments of the back. In the axis (C2), the spinous process is large and is used as a landmark in some surgical fixation techniques [4].

The Transverse Processes

The transverse processes are the paired projections that extend sideways (laterally) from the junction of the pedicle and lamina. They are attachment points for muscles and ligaments. In the thoracic region, they also carry facets for rib articulation.

Transverse processes are especially prominent in the lumbar vertebrae, where they are large, flat, and plate-like. This prominence is not cosmetic. The lumbar transverse processes serve as broad levers for the muscles that stabilize and move the lower back, and they are large enough to be used as surgical and anesthetic landmarks. In lumbar spinal anesthesia, the transverse process can be used to estimate the depth to the dural sac. A study of ex vivo trunks found that the skin-to-transverse process distance correlates with the skin-to-dural sac depth, and that the transverse process aligned with the dorsal dural sac at L3, the posterior third at L4, and the middle zone at L5 or S1 [8].

The transverse process anatomy also matters in surgery. A modified endoscopic technique for far-lateral disc herniations uses the caudal level transverse process as a docking point, which reduces the need to manipulate the exiting nerve root [9]. In the cervical region, the transverse process of C1 is a key structure in approaches to the craniocervical junction. A cadaveric study found that additional resection of the C1 transverse process mainly enhanced inferolateral working space [10].

Transverse processes can also vary in shape. An elongated transverse process of C7, measured at 24.70 mm in one case, was found in combination with a cervical rib that had fused to the first rib, a rare variation that contributed to thoracic outlet syndrome [11].

The Articular Facets

The articular facets are the paired flat surfaces where one vertebra meets the next. Each vertebra typically has two pairs: a superior pair that faces up toward the vertebra above, and an inferior pair that faces down toward the vertebra below. These facets form the zygapophyseal joints, which guide and limit movement between vertebrae. The angle and orientation of the facets change from region to region, which is one reason the cervical spine is so mobile and the lumbar spine is more restricted in rotation.

Table: Parts of a Typical Vertebra

Part	Position	Main function
Body (centrum)	Ventral, front of vertebra	Weight bearing, forms front wall of canal
Pedicle	Connects arch to body	Side wall of canal, surgical landmark
Lamina	Flat plate behind pedicle	Completes arch, protects cord
Spinous process	Midline, dorsal	Muscle and ligament attachment
Transverse process	Paired, lateral	Muscle attachment, rib facets in thorax
Articular facets	Superior and inferior pairs	Form joints between vertebrae

The Standard Mammalian Formula and the Dog

The vertebral formula is written as a series of letters and numbers, one pair for each region. For the dog, the standard formula is:

C7 T13 L7 S3 Cd variable

This means seven cervical vertebrae, thirteen thoracic vertebrae, seven lumbar vertebrae, three sacral vertebrae, and a variable number of caudal vertebrae. The cervical count of seven is nearly constant across mammals, which is why C7 appears in the formula for dogs, cats, horses, cows, humans, and ferrets alike [3].

The Anticlinal Vertebra in Dogs

In dogs, one thoracic vertebra is called the anticlinal vertebra. This is the vertebra where the spinous process changes from pointing backward (caudally) to pointing forward (cranially). It marks the transition point in the thoracic spine where the direction of the spinous processes reverses. The anticlinal vertebra is a useful landmark on radiographs because it helps orient the viewer and identify the level of a lesion. In most dogs, it is one of the vertebrae near the thoracolumbar junction, and its exact identity can vary slightly between individuals.

Comparative Vertebra Counts Across Species

The table below gives the standard regional vertebra counts for five species. Counts for the caudal region are variable and are not given as a fixed number.

Table: Regional Vertebra Counts Across Species

Species	Cervical	Thoracic	Lumbar	Sacral	Caudal
Dog	7	13	7	3	Variable
Cat	7	13	7	3	Variable
Horse	7	18	6	5	Variable
Cow	7	13	6	5	Variable
Human	7	12	5	5	Variable (fused as coccyx)

The cervical count is the same in every row. The thoracic, lumbar, and sacral counts differ. These differences reflect the different body plans and locomotor needs of each species. A horse, for example, has a long thoracic region to support a deep chest and a large set of ribs, while a human has a shorter thoracic region and a longer, more flexible lumbar region for upright posture.

The human lumbar count is worth noting because it is not fixed across all primates. The modal number of lumbar vertebrae in modern humans is five, but it varies between three and four in extant African apes, with a mean of about 3.5 [5]. This difference is part of a long-running debate about whether the last common ancestor of African apes and humans had a short or long lumbar spine [5].

How Vertebral Formulas Are Studied

Vertebral formulas are studied in several ways, and each method answers a different question.

Radiography is the most common method in veterinary practice. A study of 172 pet ferrets used radiographs to describe vertebral formulas and found five different formulas with normal vertebral morphology, plus congenital abnormalities in about 17% of the animals, mostly in the thoracolumbar and lumbosacral regions [3]. This shows that even a single species can have several normal formulas, and that transitional vertebrae are common.

Computed tomography (CT) gives finer detail. CT has been used to map the ossification centers of fetal vertebrae, including the twelfth thoracic vertebra, where the body and neural process ossification centers grow linearly with gestational age [6]. CT is also used to study the developing odontoid process in children, where the cartilaginous union at the base of the dens has its own developmental pattern [12].

Comparative and evolutionary studies use large datasets of vertebral formulas. One analysis compiled complete tetrapod vertebral formulas and searched for patterns, finding homeotic relationships in mammals and balances between distal vertebrae that were not anticipated by a simple Hox-vertebral relationship [13]. Another study in mice showed that three regulatory pathways (Gdf11, miR-196, and retinoic acid) work together to constrain total vertebral number and regional identity [2]. These findings help explain why the cervical count is so stable while the caudal count is so variable.

Comparative and Clinical Relevance

The vertebral column is not just an anatomical structure. It is the substrate for a wide range of clinical and surgical procedures, and its regional anatomy determines what is possible at each level.

In the lumbar spine, the transverse process is a key landmark for both anesthesia and surgery. For spinal anesthesia, the transverse process can be used to estimate the depth to the dural sac, and the alignment of the transverse process with the dural sac changes from L3 to S1 [8]. For pedicle screw placement, the distance from the lateral wall of the pedicle and from the angle between the transverse process and superior articular process to the inner wall of the pedicle has been measured across L1 to L5, with mean distances ranging from about 16 to 20 mm depending on the level and the measurement point [7].

In the cervical spine, the transverse process of C1 is a surgical landmark. A cadaveric study of approaches to the craniocervical junction found that far lateral exposure with condylar drilling and C1 laminectomy provided greater inferior reach, and that additional C1 transverse process resection mainly enhanced inferolateral working space [10]. This kind of detailed anatomical knowledge is what allows surgeons to reach deep structures safely.

In the thoracic spine, the transverse process is used as a target for intertransverse process blocks. A cadaveric study of injectate spread found that paravertebral spread occurred in 11 of 12 injections, and that multilevel epidural spread (three or more levels) occurred in 3 of 6 injections with 20 mL but in 0 of 6 with 10 mL [14]. This shows how vertebral anatomy directly affects the spread of local anesthetic.

In the lumbar spine, transverse process fractures have been studied as a possible predictor of pelvic ring instability. A retrospective cohort study examined whether lumbar spine transverse process fractures are significant in patients with concomitant pelvic ring injuries [15]. This is an example of how a small part of a vertebra can carry clinical meaning far beyond its size.

Common Mistakes and Limitations

Several mistakes are common when students first learn vertebral anatomy.

The first is assuming that the vertebral formula is the same for all members of a species. It is not. The ferret study found five different formulas with normal morphology in a single species [3]. Variation is the rule, not the exception, especially in the thoracic, lumbar, sacral, and caudal regions.

The second is confusing the transverse process with the spinous process. The spinous process is single and midline. The transverse processes are paired and lateral. They have different functions and different clinical uses.

The third is assuming that the cervical count can vary. In mammals, it is nearly always seven, and the ferret study found it constant in all examined animals [3]. This is one of the most reliable rules in vertebrate anatomy.

The fourth is treating the atlas and axis as typical vertebrae. They are not. They are highly modified for the special movements of the head, and they are described separately for good reason [4].

The fifth is forgetting that the caudal count is variable. Writing a fixed number for the caudal region is a mistake. It should always be described as variable unless a specific individual has been counted.

A limitation of any general guide is that individual animals and individual patients vary. A veterinarian or physician who is evaluating a specific case needs to look at that individual's imaging and history. General anatomy explains the pattern, not the particular case.

Quick Review

The vertebral column has five regions: cervical, thoracic, lumbar, sacral, and caudal.
The standard dog formula is C7 T13 L7 S3 Cd variable.
C7, seven cervical vertebrae, is nearly constant across mammals.
A typical vertebra has a body, an arch made of pedicles and laminae, a spinous process, transverse processes, and articular facets.
Transverse processes are especially prominent in lumbar vertebrae.
The atlas (C1) and axis (C2) are modified vertebrae with special shapes and functions.
The anticlinal vertebra in dogs marks the point where the spinous processes change direction.

Frequently Asked Questions

How many vertebrae does a dog have?

A dog has 7 cervical, 13 thoracic, 7 lumbar, and 3 sacral vertebrae, plus a variable number of caudal vertebrae. This is written as the formula C7 T13 L7 S3 Cd variable.

Why do almost all mammals have seven cervical vertebrae?

The cervical count is highly constrained across mammals, and seven is the near-universal number. The ferret study found the cervical count constant in all 172 animals examined, even when other regions varied [3].

What is the difference between a spinous process and a transverse process?

The spinous process is a single midline projection that points backward from the vertebral arch. The transverse processes are paired lateral projections. Both serve as muscle attachment points, but they have different positions and different clinical uses.

What is the anticlinal vertebra in dogs?

The anticlinal vertebra is the thoracic vertebra where the spinous process changes from pointing backward to pointing forward. It is a useful radiographic landmark for orienting the spine.

Are the atlas and axis typical vertebrae?

No. The atlas (C1) and axis (C2) are highly modified for head movement. The atlas is ring-shaped and allows nodding, while the axis has a dens that allows rotation.

Does the number of vertebrae differ between species?

Yes. While the cervical count is nearly always seven, the thoracic, lumbar, sacral, and caudal counts differ between species. A horse has 18 thoracic vertebrae, for example, while a dog has 13.