A set of these tubes, which can now be readily obtained, should consist of four, with the following diameters: No. 1, one centimeter; No. 2, nine millimeters; No. 3, seven millimeters; No. 4, five millimeters: their length is of course in relative and fixed proportion to these measurements. A tube should always be selected less in diameter than the trachea operated upon: to seek to introduce one of the same calibre is not only unnecessary, but cannot fail to be dangerous. Tubes constructed upon the same principles as that just described (Lüer's) are made of hard rubber instead of silver (Leiter): their lessened cost is their principal recommendation, added to the one that they are more easily kept clean and sweet than the silver tubes. The fact that they are necessarily made much heavier and thicker than the latter is a disadvantage, the lumen of a hard-rubber tube being smaller than that of a silver tube of corresponding external diameter. The objection urged against them, of their great danger of breakage, I have not found borne out by experience. Tracheal tubes are also constructed of platinum, and recommend themselves on the score of lightness. The main objection to any of the forms of tube just described exists in the nature and shape of their curve, which not infrequently causes the lower or tracheal end to lie in contact with the anterior tracheal wall, or its convexity with the posterior, and irritate, even ulcerate, them. This misfortune is entirely obviated by the canula of Durham, the second of the two forms to which I have called special attention, and which is essentially a right-angled tube, made of four sizes, with a long horizontal portion, varying from 7 to 4 centimeters, and short vertical portion, of from ½ to ¾ of an inch in length and slanting slightly backward. The former portion is capable of being lengthened or shortened in any sized tube by means of a screw arrangement attached to it as it passes through the usual neck-collar or shield; and the vertical tube can thus be correctly adapted to the particular depth at which the trachea naturally lies in a given case from the surface; and not alone this, but also to the condition of the overlying parts, whether thin or fat, swollen or otherwise. Once in position, the vertical portion of the tube remains in the long axis of the trachea, and does not touch its walls to any injurious degree. Owing to its right-angled shape, the angular and descending portions of the inner tube of this canula are necessarily made upon the lobster-tail principle, with joints--a possible disadvantage, as they can become clogged with mucus and may become detached. Other modifications and improvements exist in this Durham canula over the older one first described, which add to its utility, but need not here be dwelt upon. Suffice it to say that the tube is an excellent one for its purpose, and is deservedly highly {152} spoken of and recommended by those who have had experience in its use.