First physiology lab: mammalian body plan and comparative digestion

The following exercise involves students in a laboratory question at the beginning of an intermediate level undergraduate physiology course. Pairs of students dissect freshly killed or fresh‐frozen rats, guinea pigs or mink. With 10 pairs of students in one lab section, 4 pairs dissect mink (carnivores), 3 guinea pigs (herbivores), and 3 rats (omnivores). After following a dissection protocol to review overall internal anatomy, the students ask whether digestive functions related to diet are reflected in structures of the digestive system. They measure the lengths of small and large intestines, and determine the dimensions and weight of the caecum. They determine that, to compare digestive systems of animals of different sizes, they can express the lengths of the structures as percents of the animals’ snout‐to‐vent length. They tabulate data and computations; share data with other pairs; and produce bar graphs that show a set of data for each experimental animal. During discussion at the end of lab, the guinea pig’s voluminous caecum and relatively much longer large intestine set the stage for discussions of microbial fermentation. The mink’s simple digestive tract, and the intermediate nature of the rat’s, permit further discussion of digestive processes. This exercise accomplishes several goals: The students collect, share and present comparative data; they consider accuracy of measuring ‘slippery’ structures, how to present computations, and how to interpret published data; their observations of organs and systems in situ (often for the first time as fresh /fresh frozen tissues) prepare them for future discussions of physiology.