O

odd-toed ungulate

An order of mammals that features an odd number of toes, the middle toe is usully larger than the others. The order includes horses (Equidae), rhinoceroses (Rhinocerotidae) and tapirs (Tapiridae). They are all hind-gut fermenters with relatively simple stomachs.

oedema

A swelling that is usually painless resulting from the accumulation of fluid in tissue (i.e. fluid that is not in blood or lymph vessels). A typical sign of oedema, is that it 'pits' on pressure; digital pressure will leave a depression that remains visible for a while. In rare cases, where tissue expansion is not possible (for instance within the hoof or within bone), an oedema can cause pain.

olecranon tuberosity

A large bony prominence forming the proximal end of the ulna. The triceps brachii muscle inserts on the olecranon, this is the muscle responsible for the extension of the elbow joint.

onion heel (shoe)

A shoe with heel extensions on the inside of the heel branches. They provide protection for sensitive areas in the angle of the sole (seat of the corn), either to prevent corns from developing or to treat an existing problem.

onion heel shoe

A shoe with heel extensions on the inside of the heel branches. They provide protection for sensitive areas in the angle of the sole (seat of the corn), either to prevent corns from developing or to treat an existing problem.

onychomycosis

In human medicine, onychomycosis is a fungal disease of the toe- and fingernails. In horses, various species of fungi are often found in the crumbly, disintegrating horn typical of white line disease and seedy toe (and the hoof wall separation which results). It is as yet not certain whether these fungi are the primary cause of the disease, or are opportunistic pathogens than ‘join in’ after other factors or pathogens have caused the initial damage.

Onychomycosis is sometimes used as a synonym for one or both of the above mentioned disorders, but this is rather misleading, as it is unlikely that a fungal origin alone is responsible for the damage to the inner layers of the hoof wall.

oral administration

Administration of a substance by mouth, i.e. through the digestive tract.

oral application

Application of a substance through the mouth, i.e. through the digestive tract.

origin (of muscle or ligament)

Site at which a muscle or ligament is attached to the bone. The origin is generally more proximal, and more stable or immobile during muscle contraction when compared to the other site of attachment (the insertion).

Orohippus

About 50 million years ago at the beginning of the middle Eocene, Orohippus appeared. It was still of similar size and stature as Hyracotherium. Front limbs still ended in four, hind limbs in three toes. Over time, Orohippus' teeth showed some adaptation to a slightly coarser diet, including leaves and maybe some grasses. One of the premolars became more similar to a molar, providing more grinding surface.
During this time, the temperatures slowly started to decrease, which marked the onset of the transition in vegetation.

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