Kiddushin, Chapter One, Mishnah Four
Todays mishnah teaches how large animals such as cows and oxen, and small animals such as sheep and goats are acquired. We should note that money does not acquire animals, nor does it acquire any other movable property. According to mishnaic law a person must actually come into contact with the animal in order to acquire it. We should note that while this may have worked for the small agricultural communities of Palestine in antiquity it became very difficult by and perhaps long before the Middle Ages. By that time Jews were heavily involved in international trade and could not possibly physically handle every commodity that they acquired. Hence alternative legal means were worked out whereby money could be used to acquire legal rights to movable commodities.
Large animals are acquired by being handed over and small animals by lifting, the words of Rabbi Meir and Rabbi Elazar.
The sages say: small animals are acquired by being led.
According to Rabbi Meir and Rabbi Elazar large animals are acquired by being handed over from their owner to the purchaser. This would entail the purchaser taking hole of the reins, saddle or even hair of the animal. Small animals, however, may be lifted and therefore they are acquired only through lifting. They are not acquired by being handed over or by being led.
The sages disagree with regard to the acquisition of small animals. Despite the fact that they can be lifted, it is still difficult to do so. Therefore they are acquired by being led. Since goats and sheep do not have reins they are led by being pushed or directed by voice commands.