In the _Historia Animalium_ the homogeneous parts are divided into (1) the soft and moist (or fluid), such as blood, serum, flesh, fat, suet, marrow, semen, gall, milk, phlegm, faeces and urine, and (2) the hard and dry (or solid), such as sinew, vein, hair, bone, cartilage, nail, and horn. It would appear from this enumeration that Aristotle's distinction of simple and complex parts does not altogether coincide with our distinction of tissues and organs. We should not call vein a tissue, nor do we include under this heading non-living secretions. But in the _De Partibus Animalium_ Aristotle, while still holding to the distinction set forth above, is alive to the fact that his simple parts include several different sorts of substances. He distinguishes among the homogeneous parts three sets. The first of these comprises the tissues out of which the heterogeneous parts are constructed, _e.g._, flesh and bone; the second set form the nutriment of the parts, and are invariably fluid; while the third set are the residue of the second and constitute the residual excretions of the body (ii., 2, 647^b). He sees clearly the difficulty of calling vein or blood-vessel a simple part, for while a bloodvessel and a part of it are both blood-vessel, as we should say vascular tissue, yet a part of a blood-vessel is not a bloodvessel. There is form superadded to homogeneity of structure (ii., 2, 647^b). Similarly for the heart and the other viscera. "The heart, like the other viscera, is one of the homogeneous parts; for, if cut up, its pieces are homogeneous in substance with each other. But it is at the same time heterogeneous in virtue of its definite configuration" (ii., 1, 647^a, trans. Ogle). Aristotle, therefore, came very near our conception of tissue. He was of course not a histologist; he describes not the structure of tissues, which he could not know, but rather their distribution within the organism; his section on the homogeneous parts of Sanguinea (_Historia Animalium_, iii., second half) is largely a comparative topographical anatomy; in it, for instance, he describes the venous and skeletal systems. This distinction which Aristotle drew plays an important part in all his writings on animals, particularly in his theory of development. It was a distinction of immense value, and is full of meaning even at the present day. No one has ever given a better definition of organ than is implied in Aristotle's description of the heterogeneous parts--"The capacity of action resides in the compound parts" (Cresswell, _loc. cit._, p. 7). The heterogeneous parts were distinguished by the faculty of doing something, they were the active or executive parts. The homogeneous parts were distinguished mainly by physical characters (_De Generatione_, i., 18), but certain of them had other than purely physical properties, they were the organs of touch (_De Partibus_, ii., 1, 647^a).