It is important to note that Cuvier puts function before structure, and infers from function what the organ will be. "Plants," he writes, "having few faculties, have a very simple organisation."[43] It is only after having discussed and classified functions that Cuvier goes on to examine organs. First his views on the composition of the animal body. Aristotle distinguished three degrees of composition--the "elements," the homogeneous parts, and the heterogeneous parts or organs. Cuvier does the same. Some small advance has been made in the two thousand years' interval, due in the first place to the progress of chemistry, and in the second to the invention of the microscope. To the first circumstance Cuvier owes his knowledge that the inorganic substances forming the first degree of composition are principally C, N, H, O, and P, combined to form albumen, fibrine, and the like, which are in their turn combined to form the solids and fluids of the body. To the latter circumstance Cuvier owes the statement that the finest fragments into which mechanical division can resolve the organism are little flakes and filaments, which, joined up loosely together, form a "cellulosity." The discovery of the true cellular nature of animal tissues did not come till much later, till some years after Cuvier's death in 1832. Knowledge of histological detail was, however, considerable by the beginning of the 19th century. Cuvier knew, for example, that each muscle fibre has its own nerve fibre. But he gives no elaborate account of the homogeneous parts, no detailed histology. On the other hand his treatment of the heterogeneous parts or organs is detailed and masterly.[44] The main systems of organs are, in order of importance, the nervous and muscular, the digestive, the circulatory, and the respiratory. Each organ or system of organs may have many forms. If any form of any organ could exist in combination with any form of all the others there would be an enormous number of combinations theoretically possible. But these combinations do not all exist in Nature, for organs are not merely assembled (_rapproche's_), but act upon one another, and act all together for a common end. Accordingly only the combinations that fulfil these conditions exist in Nature. Cuvier thus dismisses the question of a science of possible organic forms and considers only the forms or combinations actually existing. This question of the possibility of a "theoretical" morphology of living things, after the fashion of the morphology of crystals with their sixteen possible types, was raised in later years by K. G. Carus, Bronn, and Haeckel.