Our previous studies have shown that Portuguese primary school pupils and teachers have three main difficulties in the representation of the digestion process: the sequence of the digestive tract, blood absorption, and the relationship of the digestive function with other human functions. In this study we analysed the topic of digestion in 63 Portuguese textbooks (1920-2005). In general, the textual explanation of the digestive process (including reference to blood absorption) is clear, but "confusion" in images is persistent throughout the period studied. To demonstrate the influence of the period of the Portuguese dictatorship (1926-1974), a brief comparison with French textbooks (which developed during the same period with the aim of presenting clearer representations of the digestive tract) was carried out. Results suggest that inadequate images in textbooks may be the main didactic obstacle to accurate learning of the digestive function. (Contains 7 tables and 5 figures.)