The Dow Jones Industrial Average midday Thursday was being yanked to session lows by a trio of components. Shares of UnitedHealth Group Inc. , Boeing Co. , and Walt Disney Co. were exacting a powerful 100-point drag on the blue-chip index, representing the lion's share of the modest declines in the price-weighted benchmark. The Dow was down 135 points, or 0.5%, at 27,089, while the S&P 500 index was sinking 0.3% lower at 2,975, with the Nasdaq Composite Index 0.5% lower at 8,143. A $1 move in any one of the Dow's components equates to a roughly 6.8-point swing in the index. UnitedHealth shares were down $6.75 at $259.90 a share, a decline of 2.5%, those for Boeing were off $5.83 to reach $363.57, a drop of 1.6%, while Disney's shares were edging 1.4% lower, off $2.03 at $140.52.