#Comedo.# (_Synonyms:_ Blackheads; Flesh-worms.) #What is comedo?# Comedo is a disorder of the sebaceous glands, characterized by yellowish or blackish pin-point or pin-head-sized puncta or elevations corresponding to the gland-orifices. #At what age and upon what parts are comedones found?# Usually between fifteen and thirty, and upon the face and upper part of the trunk, where they may exist sparsely or in great numbers. They are occasionally associated with oily seborrh[oe]a, the parts presenting a greasy or soiled appearance. Exceptionally they occur as distinct, and usually symmetrical, groups upon the forehead or the cheeks. On the upper trunk so-called double and multiple comedo have been noted--the two, three, or even four closely-contiguous blackheads are, beneath the surface, intercommunicable, the dividing duct-walls having apparently disappeared by fusion. #Describe an individual lesion.# It is pin-point to pin-head in size, dark yellowish, and usually with a central blackish point (hence the name _blackheads_). There is scarcely perceptible elevation, unless the amount of retained secretion is excessive. Upon pressure this may be ejected, the small, rounded orifice through which it is expressed giving it a thread-like shape (hence the name _flesh-worms_). #What is the usual course of comedo?# Chronic. The lesions may persist indefinitely or the condition may be somewhat variable. In many instances, either as a result of pressure or in consequence of chemical change in the sebaceous plugs or of the addition of a microbic factor, inflammation is excited and acne results. The two conditions are, in fact, usually associated. [Illustration: Fig. 8. Demodex Folliculorum, X 300. Ventral surface. (_After Simon._)] #To what may comedo often be ascribed?# To disorders of digestion, constipation, chlorosis, menstrual disturbance, lack of tone in the muscular fibres of the skin, the infrequent use of soap, and working in a dirty or dusty atmosphere. A small parasite (_demodex folliculorum_, _acarus folliculorum_) is sometimes found in the sebaceous mass, but its presence is without etiological significance, as it is also found in healthy follicles. A microbacillus has been found by several observers, and credited with etiological influence. #What is the pathology of comedo?# The sebaceous ducts or glands, or both, become blocked up with retained secretion and epithelial cells. The dark points which usually mark the lesions are probably due to accumulation of dirt, but may, as some writers maintain, be due to the presence of pigment-granules resulting from chemical change in the sebaceous matter. #Is there any difficulty in the diagnosis of comedo?# No. It can scarcely be confounded with milium, as in this latter disease the lesion has no open outlet, no black point, and the contents cannot be squeezed out. #Give the prognosis of comedo.#