the skin surface to be operated on, called the operating field, is cleaned and prepared by applying an antiseptic (ideally chlorhexidine gluconate in alcohol, as this is twice as effective as povidone-iodine at reducing the risk of infection). If hair is present at the surgical site, it is clipped off prior to prep application. The person is assisted by an anesthesiologist or resident to make a specific surgical position, then sterile drapes are used to cover the surgical site or at least a wide area surrounding the operating field; the drapes are clipped to a pair of poles near the head of the bed to form an "ether screen", which separates the anesthetist/anesthesiologist's working area (unsterile) from the surgical site (sterile). Anesthesia is administered to prevent pain from an incision, tissue manipulation and suturing. Depending on the kind of operation, anesthesia may be provided locally or as general anesthesia. Spinal anesthesia may be used when the surgical site is too large or deep for a local block, but general anesthesia may not be desirable. With local and spinal anesthesia, the surgical site is anesthetized, but the person can remain conscious or minimally sedated. In contrast, general anesthesia renders the person unconscious and paralyzed during surgery. The person is intubated and is placed on a mechanical ventilator, and anesthesia is produced by a combination of injected and inhaled agents. Choice of surgical method and anesthetic technique aims to reduce the risk of complications, shorten the time needed for recovery and minimise the surgical stress response. Intraoperative phase The intraoperative phase begins when the surgery subject is received in the surgical area (such as the operating theater or surgical department), and lasts until the subject is transferred to a recovery area (such as a post-anesthesia care unit). An incision is made to access the surgical site. Blood vessels may be clamped or cauterized to prevent bleeding, and retractors may be used to expose the site or keep the incision open. The approach to the surgical site may involve several layers of incision and dissection, as in abdominal surgery, where the incision must traverse skin, subcutaneous tissue, three layers of muscle and then the peritoneum. In certain cases, bone may be cut to further access the interior of the body; for example, cutting the skull for brain surgery or cutting the sternum for thoracic (chest) surgery to open up the rib cage. Whilst