As before mentioned, the lower air passages will not tolerate dilatation; therefore, it is necessary never to use tubes larger than the size of the passages to be examined. Four sizes are sufficient for any possible case, from a newborn infant to the largest adult. For infants under one year, the proper tube is the 4 mm. by 30 cm.; the child's size, 5 mm. by 30 cm., is used for children aged from one to five years. For children six years or over, the 7 mm. by 40 cm. bronchoscope (the adolescent size) can be used unless the smaller bronchi are to be explored. The adult bronchoscope measures 9 mm. by 40 cm. The author occasionally uses special sizes, 5 mm. x 45 cm., 6 mm. x 35 cm., 8 mm. x 40 cm. _Esophagoscopes_.-The esophagoscope, like the bronchoscope, is a hollow brass tube with beveled distal end containing a small electric light. It differs from the bronchoscope in that it has no perforations, and has a drainage canal on its upper surface, or next to the light-carrier canal which opens within the distal end of the tube. The exact size, position, and shape of the drainage outlets is important on bronchoscopes, and to an even greater degree on esophagoscopes. If the proximal edge of the drainage outlet is too near the distal end of the endoscopic tube, the mucosa will be drawn into the outlet, not only obstructing it, but, most important, traumatizing the mucosa. If, for instance, the esophagoscope were to be pushed upon with a fold thus anchored in the distal end, the esophageal wall could easily be torn. To admit the largest sizes of esophagoscopic bougies (Fig. 40), special esophagoscopes (Fig. 5) are made with both light canal and drainage canal outside the lumen of the tube, leaving the full area of luminal cross-section unencroached upon. They can, of course, be used for all purposes, but the slightly greater circumference is at times a disadvantage. The esophageal and stomach secretions are much thinner than bronchial secretions, and, if free from food, are readily aspirated through a comparatively small canal. If the canal becomes obstructed during esophagoscopy, the positive pressure tube of the aspirator is used to blow out the obstruction. Two sizes of esophagoscopes are all that are required--7 mm. X 45 cm. for children, and 10 mm. X 53 cm. for adults (Fig. 3, A and B); but various other sizes and lengths are used by the author for special purposes.* Large esophagoscopes cause dangerous dyspnea in children. If, it is desired to balloon the esophagus with air, the window plug shown in Fig. 6, is inserted into the proximal end of the esophagoscope, and air insufflated by means of the hand aspirator or with a hand bulb. The window can be replaced by a rubber diaphragm with a perforation for forceps if desired. It will be noted that none of the endoscopic tubes are fitted with mandrins. They are to be introduced under the direct guidance of the eye only. Mandrins are