But further, there are fibres of a third kind--the _oblique_--which are much fewer in number than the two kinds already spoken of. In the organs consisting of two coats this kind of fibre is found in the one coat only, mixed with the longitudinal fibres; but in the organs composed of one coat it is found along with the other two kinds. Now, these are of the greatest help to the action of the faculty which we have named _retentive_. For during this period the part needs to be tightly contracted and stretched over its contents at every point--the stomach during the whole period of digestion,[356] and the uterus during that of gestation. Thus too, the coat of a vein, being single, consists of various kinds of fibres; whilst the outer coat of an artery consists of circular fibres, and its inner coat mostly of longitudinal fibres, but with a few oblique ones also amongst them. Veins thus resemble the uterus or the bladder as regards the arrangement of their fibres, even though they are deficient in thickness; similarly arteries resemble the stomach. Alone of all organs the intestines consist of two coats of which both have their fibres transverse.[357] Now the proof that it was _for the best_ that all the organs should be naturally such as they are (that, for instance, the intestines should be composed of two coats) belongs to the subject of the _use of parts_[358]; thus we must not now desire to hear about matters of this kind nor why the anatomists are at variance regarding the number of coats in each organ. For these questions have been sufficiently discussed in the treatise "On Disagreement in Anatomy." And the problem as to why each organ has such and such a character will be discussed in the treatise "On the Use of Parts." XI Symperanteon oun hêmin ton logon heneka tôn tês alêtheias ephiemenôn hosa leipei kat' auton eti prostheisin. hôs gar hê gastêr helkei men enargôs kai kataspa ta sitia tois sphodra peinôdesi, prin akribôs en tô stomati leiôthênai, dyscherainei de kai apôtheitai tois apositois te kai pros anankên esthiousin, houtô kai tôn allôn organôn hekaston amphoteras echei tas dynameis, tên te tôn oikeiôn helktikên kai tên tôn allotriôn apokritikên. kai dia touto, kan ex henos ê chitônos organon ti synestôs, hôsper kai hai kysteis amphoterai kai hai mêtrai kai hai phlebes, amphotera tôn inôn echei ta genê, tôn eutheiôn te kai tôn enkarsiôn.