424 PHYSIOLOGY CHAP. 



uir iu the lungs can be renewed, which part may be taken as 

 corresponding to -J--1-. 



It' in the course of a series of normal quiet breathings a single 

 i nspiration of hydrogen is made, and a sample of the air collected 

 from each succeeding expiration, to ascertain which no longer 

 contains any trace of hydrogen, the result is, approximately, that 

 after 6-8 cycles the whole air of the lungs is renewed; after 

 that time every trace of hydrogen in the expired air vanishes 

 (Grehaut). 



During the entrance of air into the lungs the pressure in the 

 air-passages becomes negative ; the pressure during the exit of air 

 from the lungs, on the contrary, is positive. In order to estimate 

 these variations of intrapulmonary pressure it is only necessary 

 to connect a mercury manometer with one nostril, while the 

 mouth is kept closed, and to breathe with the other nostril 

 (Bonders). It will be seen that in quiet breathing the mercury 

 column falls 1 mm. during inspiration and rises 2-3 mm. in 

 expiration. These oscillations are increased in forced respiration. 



To ascertain the maximal values of negative and positive 

 intrapulmonary pressure, obtained by exerting all available in- 

 spiratory and expiratory forces, it is only necessary, starting from 

 zero pressure, to close the mouth and the open nostril, and then to 

 make the maximal inspiratory or expiratory effort. According to 

 Donders, the maximal negative inspiratory pressure is on an 

 average = - 57 mm. Hg (36-74) and the maximal positive expira- 

 tory pressure : + 87 mm. Hg (82-100). 



When we consider that on the one hand the inspiratory 

 muscles have to overcome great resistances in order to dilate the 

 thorax and the extensible organs which it contains, and on the 

 other the expiratory muscles are assisted in their action by the 

 same conditions which hinder the action of the former, we must 

 (notwithstanding that the manometric value of the pulmonary 

 pressure indicates a greater effect of the expiratory than of the 

 inspiratory muscles) assume that the latter develop a distinctly 

 greater amount of energy than the former. 



The oscillations of pressure along the respiratory tract may lie used in 

 man as in other animals for recording tracings of 'respiratory movements. 

 The simplest method is to introduce one end of a tube into the nasal cavity 

 or the mouth, and to attach the other to a Marey's tambour. With 

 animals it is more convenient to insert a two-way cannula into the trachea, 

 one arm of which communicates freely with the external air, while the other 

 is connected by a rubber junction with the tambour. By constricting or 

 distending the lumen of the tube through which the animal breathes, a 

 greater or less fraction of the oscillations of pressure in the trachea! air can 

 be recorded on the registering apparatus. 



A better and more exact method is that of making the animal breathe 

 into a very large vessel, communicating on the one hand with the trachea, 

 on the other with a writing tambour (Bert), or with a small and very 

 sensitive spirometer provided with a writing point that records the excursions 



