The horse sternum is a median segmental bone of its axial skeleton. It completes the skeleton of the thorax ventrally and articulates with the cartilages of the sternal ribs laterally.
Here, I will describe all the anatomical features of the horse’s sternum with diagrams.
Here, the sternum is called the segmental bone as it consists of six to seven bony segments. They are known as the sternebrae and connected by intervening cartilages.
Where is the horse’s sternum?
The figure shows the location of the horse’s sternum. It is a osteo-cartilagenious structure located at the midline of the floor of the horse’s thoracic cavity.
During the development, the costal cartilage proceeds toward the midline of the thoracic floor. Here, the extremities of the cartilages reach towards their fellows of the opposite side.
Now, the mesenchyme between the two extremities of the costal cartilage ossifies to form the equine sternum (Eydt et al., 2016)
Is the sternum boat-shaped like a horse?
Shape: The horse’s sternum is somewhat boat-shaped or canoe-shaped. It is compressed laterally, except in its posterior part.
However, the posterior part of the equine/horse’s sternum is flattened dorso-ventrally. It is inclined obliquely so that the posterior end is about six to eight inches lower than the anterior.
Horse sternum anatomy
To describe the anatomy of the horse’s sternum, you might mention the following –
- Surfaces of the sternum: Dorsal and lateral surfaces,
- Borders of the equine sternum: Dorso-lateral and ventral borders, and
- Extremities of the sternum: Anterior and posterior extremities.
Let’s discuss the osteological features from the different surfaces, borders, and extremities of the equine sternum. However, the figure provides an overview of the osteological features of the sternum.
Surfaces of the equine sternum
So, the equine sternum clearly shows two lateral and one dorsal surfaces. These surfaces are different than the sternum of the ox.
Dorsal surface: The dorsal surface or facies dorsalis has the form of a very narrow isosceles triangle. However, it possesses the apex in front. Again, it is concave longitudinally, and flattened transversely.
Lateral surfaces: The right and left lateral surfaces (facies laterales) are convex above. However, they are slightly concave below, and diminish in extent behind.
Costal cavities on the lateral surfaces: Each lateral surface presents on its upper part seven costal cavities (foveas costales). Within these costal cavities, the sternal extremities of the second to the eighth costal cartilages, inclusive, articulate.
However, these cavities are situated in series at the intersternebral junctions. Here, the first four are elliptical in outline with the long diameter vertical, and are separated by considerable regular intervals.
The others are progressively smaller, more circular, and closer together. The area below these cavities gives attachment to the pectoral muscles.
Borders of the horse’s sternum
The horse’s sternum distinctly shows two dorso-lateral borders (right and left) and one ventral border.
Dorso-lateral borders: The dorso-lateral borders separate the dorsal and lateral surfaces. They give attachment to the lateral branches of the sternal ligament.
Ventral border: The ventral border forms the prominent keel-like crest of the sternum (crista sterni). However, this structure may be felt in the living horse, and it fades out behind.
Extremities of the equine/horse’s sternum
The equine/horse’s sternum also shows the anterior and the posterior extremities. Thus, I will discuss these extremities, including their typical osteological features, with the diagrams.
Anterior extremity of the sternum
The anterior extremity or manubrium sterni can be distinctly felt in the central furrow of the breast. It consists largely of a laterally compressed cartilaginous prolongation.
This large cartilage is commonly called the cariniform cartilage. Let’s see the important features of the anterior extremity, or the manubrium of the horse.
Lateral surface: Its lateral surfaces are flat and furnish attachment to muscles of the breast and neck.
Ventral border: The ventral border of this cartilage/ manubrium is rounded. It is continued backward on the body of the sternum bone.
Dorsal border: The dorsal border of the manubrium of the equine sternum is concave.
Articular cavity: The manubrium has an articular cavity for the first pair of costal cartilages.
Posterior extremity of the sternum
The posterior extremity is formed by the xiphoid cartilage (processus xiphoideus). This is a thin plate, connected in front with the last bony segment by a relatively thick, narrow neck.
Extension: The xiphoid cartilage is expanding in nearly circular form behind and laterally.
Dorsal surface: The dorsal surface of the xiphoid cartilage is concave and gives attachment to the diaphragm.
Ventral surface: The ventral surface is convex and furnishes attachment to the transversus abdominis and the linea alba. However, the xiphoid’s free margin is very thin.
FAQ’s on the horse’s sternum
At birth, the equine sternum consists of seven bony segments termed sternebrae, which are united by intersternebral cartilages. The last two sternebrae fuse in the second month, but the others do not usually unite completely even in old age. The sternebrae consist of very vascular spongy bone covered by a very thin/narrow layer of compact substance. In older horses, these cartilages undergo partial ossification.
Well, you may easily differentiate the ox sternum from the horse’s sternum by their appearance. Here, the ox sternum is a plate-like structure that is dorsoventrally compressed except for its anterior extremity.
However, the anterior extremity of the ox sternum is laterally compressed, but not as the horse’s sternum. The manubrium and xiphoid cartilages are different in the ox compared to the horse.
The horse’s sternum has seven sternebrae, which are fused by the intersternbral cartilages. Here, the first sternebra presents an anterior cartilaginous extension.
However, the last sternebra (xiphoid cartilage) is flat and rounded, which is unique among animals.
Conclusion
So, the horse sternum is unique as it is a boat-shaped structure. However, seven sternebrae fused to form the sternum that lies along the ventral midline of the axial skeleton.
The manubrium and xiphoid cartilages of the horse’s sternum also show unique features. Here, the manubrium is laterally compressed and possesses a cartilaginous extension. However, the last segment of the xiphoid cartilage of the horse’s sternum is flat and rounded.
References
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