When personal injury is caused to an employee by accident arising out of and in the course of his employment, of which the actual or lawfully imputed negligence of the employer is the natural and proximate cause, he shall receive compensation therefor from his employer, provided the employee was himself not willfully negligent at the time of receiving such injury, and the question of whether the employee was willfully negligent shall be one of fact to be submitted to the jury, subject to the usual superintending powers of a court to set aside a verdict rendered contrary to the evidence.