CRITICAL DISCUSSION OF RESPIRATION APPARATUS. 251 



and the necessity for an occasional renewal of the material which acts as 

 a valve. It is also sometimes difficult so to adjust them that the resis- 

 tance is not only absolutely minimum but their efficiency unimpaired. 

 The membrane surrounding the valve also dries out readily on the 

 inspiring valve and must be frequently moistened. 



The Tissot valves give very satisfactory results. They are, however, 

 quite fragile, the glass part between the two metal ends breaking easily 

 and at times becoming loosened from the brass connections. Another 

 disadvantage is that the valves must be perfectly level when used, so 

 that the brass flap, which is very light and sensitive, will work properly. 

 With suitable care the valves should not get out of order. They should 

 be cleansed occasionally and the flap kept perfectly smooth to secure 

 effective closure. 



Both types of the Siebe-Gorman valves 1 are inferior to the other 

 valves mentioned and need more care when used in determining the 

 respiratory exchange during rest. 



To give efficient service, valves should offer a minimum amount of 

 resistance, close perfectly, and be easy to care for and to keep in repair, 

 so that they will be ready for use at any time. If the valves do not 

 close perfectly and there is a back-leakage of air, an actual loss may 

 result, with a consequent loss in the amount of air measured. When 

 prevention of this loss of air is made by the use of a rubber tube on 

 the intake side of the inspiratory valve, the measured volume of venti- 

 lation will tend to be greater than the true ventilation. It should 

 be pointed out that in the interpretation of results the more informa- 

 tion one has as to the character of the ventilation the more readily 

 unusual results may be interpreted. It is always advisable to safe- 

 guard the inspiratory valve by attaching a tube to the intake. 



A set of valves may be tested in two ways, for pressure and for 

 efficiency, i. e., for absence of back-leak. If in the tee-piece which 

 usually connects the ingoing and outgoing valves a side opening is 

 made large enough to insert a rubber tube approximately 3 to 4 mm. 

 in diameter, and this rubber tube is connected with an ordinary water 

 manometer, or a manometer with oil, the total variations in pressure 

 may be determined during a respiration cycle. Some fluctuations 

 in pressure are to be expected, for at the moment of inspiration there is 

 a slight vacuum in the space between the two valves and at the moment 

 of expiration there is a slight pressure. These variations, however, 

 should not be very large. A set of valves in which the fluctuations in 

 pressure exceed == 5 mm. of water is not desirable for use, as this 

 pressure is greater than would be advisable in ordinary respiration. 

 The variations in pressure may also be graphically recorded by con- 

 necting the pressure-tube to a tambour, with a pointer writing upon the 



'See figs. 29 and 30, pp. 68 and 69. 



