Vertebral formula of domestic animals

The vertebral formula of domestic animals is required for counting and identifying the vertebrae of that specific individual. Here, I will discuss the vertebral formula of domestic animals with proper diagrams.

Quick answer: The vertebral formula of domestic animals refers to expressing the number of cervical, thoracic, lumbar, sacral, and coccygeal bones in the column of individual species.

Thus, this guide helps you to know the number and differentiation of bones in the vertebral column of domestic animals. Again, you can easily prepare yourself for the exam to write the vertebral formula correctly from different animals.

Vertebral formula of domestic animals

According to specific osteological features and regions, you may easily identify the animal vertebrae in the skeleton. But, you might know the number of the vertebrae in 5 different segments of the animal’s vertebral column.

Here, Figure 1 shows the differences in the number of thoracic and sacral vertebrae from different animals. It also shows the variation in the number of the animal’s lumbar and coccygeal vertebrae.

Thus, the total number of cervical, thoracic, lumbar, sacral, and coccygeal vertebrae is different. Altogether, these vertebrae form the vertebral column in the midline of the animal’s body.

The following letters are typically used to designate the animal’s vertebrae from the  respective regions (Table 1) –

Symbol/letter	Animal’s vertebrae	Regions of the body
C	Cervical vertebrae of the animals	Neck of the animals
T	Thoracic vertebrae of the animals	Chest/thorax region
L	Lumbar vertebrae of the animals	Lion region
S	Sacral vertebrae of the animals	Pelvis region
Co/Ca	Coccygeal/caudal vertebrae	Tail of the animals

A vertebral formula for an animal consists of the letter symbol for each region, followed by the number of vertebrae of that region. The vertebral formula of domestic animals is shown in Table 2 –

Animals	Cervical	Thoracic	Lumbar	Sacral	Caudal
Cow	7	13	6	5	18 – 20
Horse	7	18	6	5	15 – 20
Sheep	7	13	6 – 7	4	16 – 18
Goat	7	13	7	4	12 – 16
Dog	7	13	7	3	20 – 24
Pig	7	14 – 15	6 – 7	4	18 – 24
Rabbit	7	12 – 13	6 – 7	4	14 – 18

How to write the vertebral formula of animals?

For your veterinary anatomy exam preparation, you might know how to write the vertebral formula of a specific animal. Well, let’s see the writing strategy of the vertebral formula of the cow, horse, goat, and sheep.

• Cow: C₇T₁₃L₆S₅Ca_18-20 
• Horse: C₇T₁₈L₆S₅Ca_15-20
• Goat: C₇T₁₃L₇S₄Ca_12-16
• Sheep: C₇T₁₃L_6-7S₄Ca_16-18   

Now, you should also write the abbreviation of the symbol/letter (C, T, L, S, Ca) with a little explanation. For example, in the case of the cow vertebral formula, you may write as below –

• Here, C₇ = seven cervical vertebrae,
• T₁₃ = thirteen thoracic vertebrae,
• L₆ = six lumbar vertebrae,
• S₅ = five sacral vertebrae, which fused to form the single sacrum, and
• Ca_18-20 = eighteen to twenty coccygeal/caudal vertebrae (depends on the tail structure and species).

Here, Figure 2 shows the typical writing strategy of the animal’s vertebral formula.

So, it is very simple to express an animal’s vertebral formula. But, in my experience, most of the students faces problem to express it perfectly. Thus, I suggest you follow this simple strategy in your veterinary anatomy examination to secure a good score.

When to explain more in writing the vertebral formula of an animal?

Well, you need to explain more while writing the vertebral formula in the situation below –

• If there are variations in the number of vertebrae in the same animal in a specific region, and
• When more vertebral bones of a specific regions fused to form a single bone.

The number of thoracic, lumbar, and caudal vertebrae varies in different animals. For example, the number of thoracic vertebrae in a horse’s vertebral column may vary from seventeen to nineteen (Robert M. Stecher, 1962). Again, pig and rabbit also present a variable number of thoracic vertebrae (14-15 and 12-13, respectively) in their vertebral column.

Thus, in this condition, you might explain why they are variable in number in the same animal. But this condition is rarely found in some animals’ vertebrae.  However, the number of caudal vertebrae varies commonly in different animals (Getty et al., 1970).

The sacral vertebrae from different animals always fuse to form a single sacrum bone. It is known as the fixed/fused vertebrae in animals. In this case, you might explain how many bones are fused and form the sacrum in the individual animal.

What is a fixed vs true vertebra in animals?

The true vertebrae in animals mean the separated, single, and movable bones of the vertebral column. For example, the first cervical (atlas) and the second cervical (axis) are the true vertebrae of animals.

Whereas, the animal’s fixed vertebrae mean the fused vertebral bones in a specific region. For example, a cow’s sacrum is a fixed vertebra of its vertebral column.

Here, Figure 3 shows the true and fixed vertebrae from the animal’s vertebral column.

What type of bones are the animal’s vertebrae?

Well, the animal vertebrae are the irregular types of bones in the skeleton. These bones do not fit into any other classification.

The animal’s vertebrae are unpaired, and their shape varies from different segments of the vertebral column. Again, they also possess irregular surfaces for muscle attachment.

They are placed at the long axis of the animal body and composed of spongy and compact substances.

Here, Figure 4 shows the osteological features of the irregular type of bones (vertebrae) from the animal’s skeleton. You may also find some of the irregular bones in the skull of the domestic animals.

Why are vertebrae different in animals?

Though the animal’s vertebrae have the basic structural plan, they vary in different segments of the column. They possess the unique features that make them different from each other.

Why: they are different because of adaptation, movement, protection, and other support to a specific region of the animal’s body.

For example, animals like cows, horses, sheep, goat can turn their neck. It is due to their movable synovial joint in the neck, which is formed by the cervical vertebrae. Thus, they are designed for adaptation to the animal’s neck anatomy.

The body and transverse process of thoracic vertebrae possess facets for articulation with the proximal end of ribs. They also form the synovial joint but permit slight gliding and rotatory movement.

Thus, the thoracic vertebrae are different as their main role is to form articulation with the ribs.

Again, the animal’s lumbar vertebrae have a well-developed articular surface. They support the lion and help in movements.

However, the sacral vertebrae are totally different from other vertebrae. It is because they have to support and form the floor of the animal’s pelvic cavity.

Finally, the caudal vertebrae are different from others to support the back region of animals.

Here, Figure 5 shows why vertebrae are different in the 5 segments of the animal’s vertebral column.

FAQ’s on vertebral formula in animals

Conclusion

So, the vertebral formula of domestic animals is expressed with the symbols/letters C, T, L, S, and Ca. Here, the letter C, T, L, S, and Ca denote the cervical, thoracic, lumbar, sacral, and coccygeal vertebrae with their specific number in specific animals.

The ideal vertebral formula for a cow is C₇T₁₃L₆S₅Ca_18-20, whereas a horse is C₇T₁₈L₆S₅Ca_15-20. Thus, it means the vertebral formula is animal-specific.

References

• Robert M. Stecher, Anatomical Variations of the Spine in the Horse, Journal of Mammalogy, Volume 43, Issue 2, 29 May 1962, Pages 205–219,
• Nourinezhad et al., Morphometric development of the vertebral segments of the gpat’s vertebral column. Int. J. Morphol., 35(2):506-514, 2017.
• Proks et al., Vertebral formula and abnormalities of the vertebral column in rabbits, The Veterinary Journal, Volume 236, 2018, Pages 80-88, ISSN 1090-0233, https://doi.org/10.1016/j.tvjl.2018.04.016. 
• Spoormakers et al., Comparative study of anatomical configuration of the equine vertebral column. Journal of Anatomy, 2021; 239(4), 829–838. https://doi.org/10.1111/joa.13456 
• Ghosh, R. K. : Primary veterinary anatomy, 2nd Edn.Current Books International, Kolkata.
• Rowen D. Frandson, W. Lee Wilke, and A. Dee: Anatomy and physiology of farm animals, 4th edn.Wiley-Blackwell, USA.
• Getty, R,. S&G, Anatomy of the domestic animals, 5th Edn. W. B. Saynders and Co. Philadelphia (USA)