DEGREES OF COMPOSITION 13 



The second degree of composition is that by which the 

 homogeneous parts of animals, such as bone, flesh, and the 

 like, are constituted out of the primary substances. The 

 third and last stage is the composition which forms the 

 heterogeneous parts, such as face, hand, and the rest" (ii., I, 

 646 a , trans. Ogle). 



In the Historia Aniinalium the homogeneous parts are 

 divided into (i) 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 Aniinalium 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 homo- 

 geneous 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 blood- 

 vessel 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 blood- 

 vessel. 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 homo- 

 geneous 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., i, 

 647^ 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 Aniinalium, iii., 



