ms:_ 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 po