Head: The front portion of the dog, including the muzzle and the cranium.

Head Planes: Viewed in profile, the contours of the top portion of the skull from occiput to stop, and the foreface from stop to tip of the nose.

Heartworm: A filarial worm (Dirofilaria immitis) transmitted by mosquitoes and parasitic in the heart and associated blood vessels of dogs and other canids.

Heat: (1) Seasonal period of the female. Estrus, in season. (2) Performance: A competitive running of dogs.

Heel: Command to a dog to keep close beside its handler.

Height: Vertical measurement from the withers to the ground; usually called shoulder height.

Hemorrhage: Excessive discharge of blood from the blood vessels; profuse bleeding.

Hepatitis: Inflammation of the liver, caused by infectious or toxic agents and characterized by jaundice, fever, liver enlargement, and abdominal pain.

Herding Group: Group of dogs bred to help shepherds and ranchers herd or work livestock.

Hie On: A command to urge the dog on; used in hunting or in field trials.

High Standing: Tall and upstanding, with plenty of leg.

Hindquarters: Rear assembly of the dog (pelvis, thighs, hocks, and paws).

Hip Dysplasia: Abnormal formation of the hip joint.

Histamine: A physiologically active amine, C5H9N3, found in plant and animal tissue and released from mast cells as part of an allergic reaction in humans. It stimulates gastric secretion and causes dilation of capillaries, constriction of bronchial smooth muscle, and decreased blood pressure.