Now Nature has made no organ for clearing away _phlegm_, this being cold and moist, and, as it were, half-digested nutriment; such a substance, therefore, does not need to be evacuated, but remains in the body and undergoes _alteration_ there. And perhaps one cannot properly give the name of _phlegm_ to the surplus-substance which runs down from the brain,[297] but one should call it _mucus_ [blenna] or _coryza_--as, in fact, it is actually termed; in any case it will be pointed out, in the treatise "On the Use of Parts," how Nature has provided for the evacuation of this substance. Further, the device provided by Nature which ensures that the phlegm which forms in the stomach and intestines may be evacuated in the most rapid and effective way possible--this also will be described in that commentary. As to that portion of the phlegm which is carried in the veins, seeing that this is of service to the animal it requires no evacuation. Here too, then, we must pay attention and recognise that, just as in the case of each of the two kinds of bile, there is one part which is useful to the animal and in accordance with its nature, while the other part is useless and contrary to nature, so also is it with the phlegm; such of it as is sweet is useful to the animal and according to nature, while, as to such of it as has become bitter or salt, that part which is bitter is completely undigested, while that part which is salt has undergone putrefaction. And the term "_complete indigestion_" refers of course to the second digestion--that which takes place in the veins; it is not a failure of the first digestion--that in the alimentary canal--for it would not have become a humour at the outset if it had escaped this digestion also. It seems to me that I have made enough reference to what has been said regarding the genesis and destruction of humours by Hippocrates, Plato, Aristotle, Praxagoras, and Diocles, and many others among the Ancients; I did not deem it right to transport the whole of their final pronouncements into this treatise. I have said only so much regarding each of the humours as will stir up the reader, unless he be absolutely inept, to make himself familiar with the writings of the Ancients, and will help him to gain more easy access to them. In another treatise[298] I have written on the humours according to Praxagoras, son of Nicarchus; although this authority makes as many as ten humours, not including the blood (the blood itself being an eleventh), this is not a departure from the teaching of Hippocrates; for Praxagoras divides into species and varieties the humours which Hippocrates first mentioned, with the demonstration proper to each.