In this photograph from a French textbook, the doctor is introducing a rigid endoscope into the patient's mouth in order to examine the throat. With the patient in this position, it would be impossible to pass a rigid scope into the oesophagus (tube leading from the throat to the stomach). This could only be achieved with the patient lying on his back and neck extended in order to ‘straighten' the line between mouth and stomach.

Source: François Moutier. Traite de gastroscopie ... Masson & Cie, Paris 1935. Reproduced with kind permission of the publisher.