This is a caricature of a patient opening wide to a dentist, so wide that the dentist can see and examine some of his internal organs. It is funny but there is some truth into it.
When someone has a systemic disease, sometimes it can have oral signs. Like for example diabetes. If the dentist suspects anything, he or she will refer the patient to the general doctor to have it checked.
Also they say that a healthy mouth is a sign of a healthy body. That’s why it’s important to keep your teeth and gums healthy in order not to cause infection or inflammation that can reach other organs of your body like your heart and your lungs.