But when the flesh wastes and returns into the veins there is
discoloured blood as well as air in the veins, having acid and
salt qualities, from which is generated every sort of phlegm and
bile. All things go the wrong way and cease to give nourishment
to the body, no longer preserving their natural courses, but at
war with themselves and destructive to the constitution of the
body. The oldest part of the flesh which is hard to decompose
blackens from long burning, and from being corroded grows bitter,
and as the bitter element refines away, becomes acid. When tinged
with blood the bitter substance has a red colour, and this when
mixed with black takes the hue of grass; or again, the bitter
substance has an auburn colour, when new flesh is decomposed by
the internal flame. To all which phenomena some physician or
philosopher who was able to see the one in many has given the
name of bile. The various kinds of bile have names answering to
their colours. Lymph or serum is of two kinds: first, the whey of
blood, which is gentle; secondly, the secretion of dark and
bitter bile, which, when mingled under the influence of heat with
salt, is malignant and is called acid phlegm. There is also white
phlegm, formed by the decomposition of young and tender flesh,
and covered with little bubbles, separately invisible, but
becoming visible when collected. The water of tears and
perspiration and similar substances is also the watery part of
fresh phlegm. All these humours become sources of disease when
the blood is replenished in irregular ways and not by food or
drink. The danger, however, is not so great when the foundation
remains, for then there is a possibility of recovery. But when
the substance which unites the flesh and bones is diseased, and
is no longer renewed from the muscles and sinews, and instead of
being oily and smooth and glutinous becomes rough and salt and
dry, then the fleshy parts fall away and leave the sinews bare
and full of brine, and the flesh gets back again into the
circulation of the blood, and makes the previously mentioned
disorders still greater. There are other and worse diseases which
are prior to these; as when the bone through the density of the
flesh does not receive sufficient air, and becomes stagnant and
gangrened, and crumbling away passes into the food, and the food
into the flesh, and the flesh returns again into the blood. Worst
of all and most fatal is the disease of the marrow, by which the
whole course of the body is reversed. There is a third class of
diseases which are produced, some by wind and some by phlegm and
some by bile. When the lung, which is the steward of the air, is
obstructed, by rheums, and in one part no air, and in another too