**Core Concept**
A Gridiron incision is a type of surgical incision used in abdominal surgery, specifically for accessing the peritoneal cavity. It is characterized by a series of parallel incisions in the rectus sheath, allowing for minimal disruption to the abdominal wall. Rutherford Morison's incision is an extension of this technique, providing a more extensive exposure for surgical procedures.

**Why the Correct Answer is Right**
The correct answer, **B. Cutting the muscles laterally**, is a critical step in converting a Gridiron incision to a Rutherford Morison's incision. By cutting the muscles laterally, the surgeon creates a wider exposure, allowing for better access to the abdominal contents. This is essential for extensive surgical procedures, such as those involving the liver, spleen, or kidneys. The lateral cutting of muscles also helps to minimize the risk of wound complications and promotes healing.

**Why Each Wrong Option is Incorrect**
**Option A:** Splitting the muscles laterally is incorrect because it would not provide the necessary exposure for a Rutherford Morison's incision. Splitting implies a separation of the muscle fibers, whereas cutting laterally involves a more aggressive incision through the muscle.
**Option C:** Cutting the muscles medially into the rectus sheath is incorrect because it would compromise the integrity of the rectus sheath, leading to potential herniation and wound complications. Medial cutting would also limit the exposure, making it unsuitable for a Rutherford Morison's incision.
**Option D:** Incising vertically along the rectus muscle is incorrect because it would not provide the necessary lateral exposure required for a Rutherford Morison's incision. Vertical incisions would also increase the risk of wound complications and limit the surgeon's access to the abdominal contents.

**Clinical Pearl / High-Yield Fact**
When converting a Gridiron incision to a Rutherford Morison's incision, it is essential to maintain a clear understanding of the anatomy and the surgical goals. This involves careful planning and execution to ensure optimal exposure and minimize complications. A Rutherford Morison's incision is typically used for extensive procedures, such as liver resections or pancreatic surgery, where maximal exposure is necessary.

**✓ Correct Answer: B. Cutting the muscles laterally**