Overview
The horn (cornu) is a modified skin structure made of a bony core covered by specialised skin. Both sexes can have horns. Polled breeds lack horns because of a dominant inherited trait.
Structure
- Cornual process (processus cornualis): extension of the frontal bone that forms the core of the horn. After the first months of life it becomes pneumatised by a diverticulum of the frontal sinus, so that its cavity is continuous with the sinus.
- Horn dermis: vascular, innervated connective tissue attached to the periosteum of the cornual process; it nourishes the epidermis that produces the horn.
- Horn sheath: sheath of hard keratin made of horn tubules. It grows continuously from the base, where the horn skin is continuous with the haired skin, and moves towards the tip.
Development
In the calf the horn starts as a horn bud, an area of specialised skin over the frontal bone that is not yet attached to it. As the calf grows, the underlying bone forms the cornual process and the horn becomes fixed to the skull.
Clinical relevance
- Disbudding and dehorning: horns are removed to reduce injuries between animals and to people. Disbudding of calves destroys the horn bud, usually with heat; dehorning of adults cuts through the bony process and is more traumatic. Both require local anaesthesia and analgesia.
- Cornual nerve block: the cornual nerve, a branch of the trigeminal nerve, supplies most of the horn. It is blocked along the temporal line of the frontal bone, between the lateral angle of the eye and the base of the horn.
- Frontal sinusitis: dehorning an adult leaves the frontal sinus open; contamination of the wound can cause sinusitis, sometimes weeks or months later.
- Horn fractures: they can expose the sinus and bleed heavily.
- Horn rings: transverse grooves in the horn sheath that reflect changes in growth. They have been linked to calvings and are sometimes used to estimate age, but they are not a reliable indicator.