The navicular bones are the distal sesamoids in animals that are located between the middle and distal phalanges. This guide will help you to learn the detail anatomical features of the navicular bone in animals with diagrams.
Quick answer: The navicular bone in animals is the flattened distal sesamoid placed between the middle and distal phalanges of both limbs.
Sometimes, they are called the third sesamoid bone in the horse. The number of the navicular bones in a limb is animal-specific.
Navicular bone in animals
There are two types of sesamoid bones in an animal’s limbs – proximal and distal. The distal, smaller, seed-like or seasame bone is known as the navicular bone in animals.
Both the proximal and distal sesamoid or navicular bones develop within the tendon. They ossify after birth and work as a pully to avoid friction in the complex joints of an animal’s forelimbs and hindlimbs.
Here, Figure 1 shows both the proximal and distal sesamoids from the animal’s limbs.
Number of navicular bones in animals
Number of navicular bones in a cow: In a cow, there are two in number for each limb, one for each digit. Thus, you will find 8 (eight) navicular bones in the cow’s four limbs (2 forelimbs and 2 hindlimbs).
The counting of distal sesamoid or navicular bones in a cow is shown in Table 1 –
| Limbs and sesamoid bones | Number count |
| Forelimb in a cow Developed digits in the forelimb Distal sesamoid (navicular) in each digit | 2 2 1 |
| Thus, the total navicular in the cow’s forelimb | [(1×2) x 2] = 4 |
| Hindlimb in a cow Developed digits in the hindlimb Distal sesamoid (navicular) in each digit | 2 2 1 |
| Thus, the total navicular in the cow’s hindlimb | [(1×2) x 2] = 4 |
| Total navicular bones in a cow | 4 + 4 = 8 |
Explanation of [(1×2) x 2] = 4: Here, 1 = one navicular bone for each digit; 2 = two developed digits in each forelimb or hindlimb; and 2 = right and left forelimb or hindlimb.
In simple terms, each developed digit of a cow contains 1 navicular bone. Again, the cows have 4+4 = 8 developed digits in both forelimb and hindlimb. Thus, the total number of navicular bones in a cow is 8 (eight).
Number of navicular bones in a horse
There are a total of 4 navicular bones in a horse. This is because the horse has only one developed digit in each limb. Thus, the 4 limbs of a horse (2 forelimbs and 2 hindlimbs) contain 4 navicular bones.
Table 2 shows the counting of the navicular bones in a horse –
| Limbs and sesamoid bones in a horse | Number count |
| Forelimb in a horse Developed digits in the forelimb Distal sesamoid (navicular) in each digi | 2 1 1 |
| Thus, the total navicular in the horse’s forelimb | [(1×1) x 2] = 2 |
| Hindlimb in a horse Developed digits in the hindlimb Distal sesamoid (navicular) in each digit | 2 1 1 |
| Thus, the total navicular in the horse’s hindlimb | [(1×1) x 2] = 2 |
| Total navicular bones in a horse | 2 + 2 = 4 |
Where is the navicular bone in animals?
The navicular bone is located transversely behind the junction between the middle and distal phalanges in animals. This joint is also known as the distal interphalangeal or coffin joint.
Here, Figure 2 shows the location of the navicular bones in both forelimb and hindlimb of the animals.
Navicular bone anatomy in animals
The navicular bone of a cow is short and flattened craniocaudally. For description purposes, the navicular bone possesses 2 surfaces, 2 borders, and 2 extremities.
- Surfaces: they are cranial or articular surfaces, and caudal or flexor or tendon surface.
- Borders: they are the proximal border (dorsal) and the distal border (ventral), and
- Extremities: they are the medial and lateral borders,
Figure 3 shows the surfaces, border, and extremities of an animal’s navicular bone.
Surfaces, borders, and extremities of the navicular bone
The articular surface faces dorsally and cranially. It possesses a central prominence, flanked by a concave area. This surface articulates with the distal extremity of the middle or second phalanx of the animal’s digit.
The flexor surface of the navicular bone directs downward and backward. It resembles the articular surface in form, but is more extensive and not smooth in appearance.
This flexor surface is broader and covered by a fibrocartilaginous structure for the passage of the tendon of insertion (TN) of the deep digital flexor.
The proximal border (PB) of the navicular bone in animals is wide and grooved in its middle. The ligament attaches to this groove of the proximal border. This surface is also narrow and rounded on either side.
The distal border of the navicular bone possesses a narrow facet on its cranial aspect. This facet is design for articulation with the proximal extremity of the distal phalanx.
Behind the groove on the distal border, you will find relatively larger foramina. Again, there is a ridge that bounds the groove caudally.
Both the medial and lateral extremities of the navicular bone are rounded and smooth.
How will you differentiate the navicular bone from the proximal sesamoids?
The navicular bone can be differentiated from the proximal sesamoids by their shape. The proximal sesamoid bone in animals has the form of a three-sided pyramid. But the shape of the navicular bone is short and craniocaudally flattened.
The proximal sesamoid bone has 3 surfaces (articular, flexor, abaxial), a base, and an apex. Again, the navicular bone possesses 2 surfaces (articular and flexor), 2 borders (proximal and distal), and 2 extremities (medial and lateral).
You will find the difference in the number and location of the proximal sesamoid compared to the navicular bones. There are 2 proximal sesamoid bones for each digit in most animals.
Again, the proximal sesamoid bone is located behind the junction of the distal extremity of the metacarpal or metatarsal and the proximal phalanx. This junction of these bones is known as the metacarpophalangeal articulation or fetlock joint.
Figure 4 shows the difference between the proximal and distal sesamoid bones of animals.
Special features of the navicular bone
The navicular bone has no haversian system and periosteum, like the normal bone of the animal’s body. They are design to reduce friction in the complex joints of the animal’s limbs, especially in digits.
The navicular or distal sesamoid bone forms the distal interphalangeal articulation along with the metacarpal and proximal phalanx. It is also known as the coffin joint and is largely encased within the hoof. This coffin or distal interphalangeal articulation is a ginglymus-type joint in animals.
Conclusion
So, the navicular bone in animals is the distal sesamoid, which is located between the middle and distal phalanges. There are two in number, each for one digit in cow.
The navicular bone possesses a convex articular surface and an almost flat flexor surface, along with 2 borders and extremities.
References
- Larsen et al., 2023. Navicular bone fracture and severe deep digital flexor tendinopathy after palmar digital neurectomy in two horses. Open Veterinary Journal, 13 (12).
- 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,. Sission and Grossman’s The anatomy of the domestic animals, 5th Edn. W. B. Saynders and Co. Philadelphia (USA).
- Nickel, R. Schummer, A. and Seiferle, E.: The anatomy of domestic animals, 1st Edn. Verlag Paul Parey, Berlin.
