4. SAFETY VALVE.--Dr Meltzer insists very strongly on the necessity of having a safety valve capable of controlling the maximum pressure under which the air may enter into the intratracheal tube. By a simple device, any excess of desired pressure will cause the air to bubble through mercury and thus never reach the lungs. In this way any possibility of accident from undue intrapulmonary pressure is obviated. [Illustration: FIG. 33.--Shipway’s intratracheal apparatus.] 5. MERCURY MANOMETER to indicate the pressure in the tube in m.m. of mercury. It has been experimentally established[7] “that the pressure in the trachea and in the bronchi is only a small fraction of the pressure in the manometer outside of the body, and that the intratracheal pressure grows considerably less with the decrease of the diameter of the intratracheal tube.” 6. INTRATRACHEAL CATHETER.--It is essential to have an instrument of adequate rigidity which can be satisfactorily sterilized. The ordinary coudé catheters, or the silk web white enamelled cylindrical catheters, are suitable. The size should be selected from a range of 18 to 25 French. It is preferable to err rather on the side of a small than a large tube. An ordinary adult will require a tube of about size 22 to 24, a plethoric alcoholic, on the other hand, might need a 25. A tube of too large a calibre interferes with the free return of air and spontaneous respiration soon becomes too slow. Expiration is prolonged, active, and laboured, and after a few minutes, respiratory movements may cease entirely. The only way to meet such a situation is to withdraw the tube and insert a smaller one. TECHNIQUE OF ADMINISTRATION.--It is advisable in adults to administer about three quarters of an hour before the operation, a hypodermic injection of morphia (gr. ⅙) and atropine (gr. ¹⁄₁₀₀), or of scopolamine (gr. ¹⁄₁₀₀) and morphia (gr. ⅙). The latter combination is more efficacious in alcoholic subjects. In children, atropine alone should be given. It is to be remembered that intratracheal insufflation of ether is a method of maintaining not of inducing anæsthesia. Induction is carried out in the ordinary way. When this has been done, the catheter is passed. The introduction of the catheter does present some difficulty, but this is largely overcome as skill and confidence are acquired with practice. It may be carried out indirectly, or a view of the glottis may be obtained by the aid of such an endoscope as Hill’s, and the catheter inserted between the cords. The latter method is probably the more satisfactory, but it is well to acquire the skill to pass the catheter indirectly as in a certain small proportion of cases there are obstacles to the use of the endoscope.