blackhead is the common name for a type of comedo(ne), the characteristic feature of acne vulgaris. They occur mostly on the face, neck, and upper part of the back, because this is where sebaceous glands are most thickly distributed. These glands secrete sebum, a fatty lubricant, into the hair follices. The lining of the follicles, like the skin surface itself, continually sheds and renews its outermost layer, and this debris, along with the sebum, normally escapes onto the surface. When the lining fails to be shed properly the sebum accumulates below it in the hair follicle. The debris of skin cells, pushed outwards by the sebum contains the pigment melanin, and this accounts for the dark plug that closes off the pore. Inflammation may be caused by bacterial action and fatty acids formed in the sebum, resulting in papules (lumps) and pustules (‘boils’ or ‘plukes’). Male sex hormones enhance the activity of the sebaceous glands along with the stimulation of hair growth, accounting for their exuberant secretion, and hence the likelihood of acne, around the time of puberty, and for the fact that girls suffer less — though they are not entirely exempt.

Stuart Judge

Blackheath, battle of, 1497. A formidable rising of Cornishmen in the summer of 1497 protested against taxation to support Henry VII's campaign against James IV of Scotland, arguing that this was a purely northern responsibility. Under the leadership of Thomas Flammock and James, Lord Audley, the rebels marched through Wells, Salisbury, and Winchester towards London, where they caused great panic. But Lord Daubeney and the earl of Oxford led the army prepared for Scotland against the insurgents at Blackheath and dispersed them. Flammock and Audley were executed.

J. A. Cannon

blackhead (blak-hed) n. a plug formed of fatty material (sebum and keratin) in the outlet of a sebaceous gland in the skin. See also acne. Medical name: comedo.

black·head / ˈblakˌhed/ • n. a plug of sebum in a hair follicle, darkened by oxidation.

blackhead A disease of turkeys caused by the protozoon Histomonas meleagridis. The disease affects the caecum and liver of the bird, and the mortality rate may be high. The disease is transmitted by a nematode worm parasitic in the intestines of turkeys.