Mumps is an acute, contagious, systemic viral disease caused by a paramyxovirus. It is spread by droplets or saliva and probably enters through the nose and mouth. After a 12- to 24-day incubation period, headache, anorexia, malaise, and low-grade fever usually develop. Then several other symptoms develop over the next few days. Of these symptoms, edema of which of the following structures is most likely to peak on the 2nd day and typically last 5 to 7 days?