PRESSURE IN LUNGS AND PLEURAL CAVLTLES. 281 



Hence, as the chest expands from the expiratory to the inspiratory 

 position, the lower border of the lungs moves from the level of the 

 sixth or seventh rib to that of the eleventh. There is, moreover, a 

 movement of the anterior margins of the lungs forward, consequent on 

 the lateral expansion of these organs. On this account the area of 

 cardiac dulness is markedly decreased, or even abolished, during a deep 

 inspiration. 



On applying the ear to the chest of a healthy individual, a delicate 

 rustling sound is heard with each inspiration. This sound is known as 

 the " vesicular murmur," and is supposed to be due to the entry of air 

 into the alveoli. Expiration is either noiseless or is accompanied by a 

 much fainter murmur of the same character, which only lasts during the 

 first third of the expiration. If we auscultate over the trachea or 

 bronchi, a sound is heard which is quite distinct in character from the 

 vesicular murmur, and is known as " bronchial breathing." It is a loud 

 blowing murmur, resembling the sound produced by approximating the 

 root of the tongue to the soft palate and expiring, as in pronouncing the 

 Greek letter y\ It differs from the vesicular murmur, moreover, in that 

 the expiratory part is as loud as or louder than the inspiratory. It is 

 probably generated in the larynx, since complete division of the trachea 

 below the glottis abolishes the sound. 



Respiratory rhythm — In normal conditions each inspiration is 

 followed immediately by an expiration, which is succeeded by a slight 

 pause before the next inspiratory movement takes place. The inspira- 

 tion is generally shorter than the expiration, the time occupied by the twu 

 phases being in the proportion 10 to 12. The pause between two respira- 

 tions lasts about one-third to one-fifth of the total time between the 

 beginning of one inspiration and the next. 



Both the frequency of respiration and the amount of air respired 

 vary within wide limits in different individuals, and under various con- 

 ditions ; these are fully discussed in the article on the " Chemistry of 

 Respiration " in Vol. I. 



Pressure in the lungs and pleural cavities — We have already seen 

 that, even in conditions of expiration, the lungs are still in a stretched 

 condition, and therefore exert a pull on the thoracic walls, until they 

 are allowed to collapse by puncture of the wall. The amount of this 

 elastic force has been measured by connecting a mercurial manometer 

 with the trachea in the dead body, and then puncturing the thorax. 

 Measured in this way it amounts to between 2 and 5 mm. Hg. Accord- 

 ing to Donders, 1 however, this is less than the actual pressure in the 

 living body, owing to a loss of elasticity by the lungs. Donders 

 calculated the elastic pull of the lungs in the expiratory condition as 

 about 7'5 mm. Hg. After an ordinary inspiration this is increased to 

 9 mm., and after the deepest possible inspiration to about 30 mm. It 

 is evident that these figures also represent the negative pressure in the 

 thoracic cavity under the same conditions. 



During ordinary respiratory movements there are slight changes of 

 pressure in the alveoli and tracheal air. In inspiration the pressure in 

 the trachea is from - 2 to - 3 mm. Hg, in expiration about + 2 mm. Hg. 

 By connecting the closed trachea or air passages with a manometer, we 

 may measure the total possible force exerted by the respiratory 

 muscles in drawing air in or expelling air from the lungs. These 

 1 Ztsckr. f. rut. Med., 1853, N". F., Bd. iii. (quoted from Rosenthal, loc. cit.). 



