MEXICO CITY, Aug 24 (Reuters) - Mexico"s national immigration institute (INM) said on Tuesday that one of its agents was hospitalized in a serious condition after being struck in the face and head with rocks a day earlier by a group of migrants. The INM said its agents were attacked by the group of migrants who were walking along a rail line in the municipality of Apizaco in the Mexican state of Tlaxcala in an effort to impede the agents from verifying their migration status. The injured agent was transferred to a brain surgery ward on Tuesday evening, the INM said. Tens of thousands of people, many from Central America, traverse Mexico every year in an effort to cross into the United States.