Now the peculiar[42] flesh of the liver is of this kind as well, also that of the spleen, that of the kidneys, that of the lungs, and that of the heart; so also the proper substance of the brain, stomach, gullet, intestines, and uterus is _a sensible element_, of similar parts all through, simple, and uncompounded. That is to say, if you remove from each of the organs mentioned its arteries, veins, and nerves,[43] the substance remaining in each organ is, from the point of view of the senses, simple and elementary. As regards those organs consisting of two dissimilar _coats_,[44] of which each is simple, of these organs the coats are the elements--for example, the coats of the stomach, oesophagus, intestines, and arteries; each of these two coats has an alterative faculty peculiar to it, which has engendered it from the menstrual blood of the mother. Thus the _special_ alterative faculties in each animal are of the same number as the elementary parts[45]; and further, the _activities_ must necessarily correspond each to one of the special parts, just as each part has its special _use_--for example, those ducts which extend from the kidneys into the bladder, and which are called _ureters_; for these are not arteries, since they do not pulsate nor do they consist of two coats; and they are not veins, since they neither contain blood, nor do their coats in any way resemble those of veins; from nerves they differ still more than from the structures mentioned. "What, then, are they?" someone asks--as though every part must necessarily be either an artery, a vein, a nerve, or a complex of these,[46] and as though the truth were not what I am now stating, namely, that every one of the various organs has its own particular substance. For in fact the two bladders--that which receives the urine, and that which receives the yellow bile--not only differ from all other organs, but also from one another. Further, the ducts which spring out like kinds of conduits from the gall-bladder and which pass into the liver have no resemblance either to arteries, veins or nerves. But these parts have been treated at a greater length in my work "On the Anatomy of Hippocrates," as well as elsewhere. As for the actual substance of the coats of the stomach, intestine, and uterus, each of these has been rendered what it is by a special alterative faculty of Nature; while the bringing of these together,[47] the combination therewith of the structures which are inserted into them, the outgrowth into the intestine,[48] the shape of the inner cavities, and the like, have all been determined by a faculty which we call the shaping or formative faculty[49]; this faculty we also state to be _artistic_--nay, the best and highest art--doing everything for some purpose, so that there is nothing ineffective or superfluous, or capable of being better disposed. This, however, I shall demonstrate in my work "On the Use of Parts." VI