A span of mules

In Missouri, where they raise more mules than in any other place in the world, a farmer died, leaving behind seventeen mules and three sons. In his will he disposed of his mules as follows: One-half to the eldest son, one-third to the next, and one-ninth to the youngest.
The administrator who went to divide the property drove a span of mules out to the farm, but when he went to divide the seventeen into halves, thirds and ninths he found it was impossible with live mules. Mules not being very valuable at the time, he unhitched one of his own, putting it with the seventeen, making eighteen, which he proceeded to divide as follows:
One-half, or nine, to the eldest son.
One-third, or six, to the next son.
One-ninth, or two, to the youngest.
Adding up nine, six, two, he found that it made seventeen, so he hitched up his own mule and went away rejoicing.

Could you tell me what "a span of mules" means?