According to the Mishna (daf 42a) if an animal fell from a roof it is a tereifa, since we fear that its limbs were shattered or internal organs were injured by the fall. The simple reading of the Mishna appears to rule that even if there was no external evidence of injury, nevertheless we must make this assumption if the animal was slaughtered immediately. From the Mishna later on in the perek (see daf 56a) it is clear that if they wait a 24-hour period before slaughter, the animal is kosher, as we can be certain that there was no serious internal injury.
On today’s daf the Gemara quotes Rav Huna as teaching that if an animal was left on the roof and was later found on the ground, we do not assume that it fell and the ruling of the Mishna is not applied to it. To illustrate and clarify this ruling, the Gemara relates the following story:
A goat belonging to Ravina was on the roof and through the sky-light saw some peeled barley below. It jumped and fell down from the roof to the ground. Ravina came before Rav Ashi and asked: Was the reason for Rav Huna’s statement, ‘If a person left an animal on the roof, and returned and found it on the ground we need not be concerned about the shattering of limbs,’ because the animal had something to grab hold of, but in this case it had nothing to grab hold of; or was it that the animal estimated the distance, so that here too it estimated the distance? He replied. The reason was that it estimated the distance; so that here too it estimated the distance and it is therefore permitted.
The Rashba concludes from this story that the ruling of the Mishna is limited to cases where the animal fell, but it does not extend to cases where the animal chose to jump, since in such cases the animal gauges whether it will be injured. Furthermore, if we do not know how the animal reached the ground we do not assume that it fell – which is an unusual occurrence – rather that it jumped. According to the Rashba, there is no need for any further assessment of the animal, even if it no longer stands, but other rishonim argue that even if the concern with internal injury does not apply, still we must worry that some other injury may have taken place that might render the animal a tereifa, since we know that even a simple fall suffered by a human being can sometimes lead to death.