The intrinsic eye muscles, or the <span>sphincter pupillae.</span>
<span>the word that the appositive renames or further identifies - agent</span>
The t-shirt that they give only to people who finish is a big reason that so many people take up the challenging race.
This paraphrase presents the same information as in the sentence, but since it should be a bit more of a summary, it takes out the specific detail of how many people actually run the race, which can just be paraphrased into 'so many'.
