BY DR. BURDON-SANDERSON. 303 



of the chest more than of the lower part, so that the normal 

 relation between the two is reversed. 



90. Measurement of the Intra-Thoracic Pressure. 

 In consequence of the elasticity of the lungs, and of the 

 fact that they are contained in a cavity of which the capacity 

 is much greater than the volume which these organs assume 

 in their unextended condition, and that their external surface 

 is inseparably applied to the inner surface of the cavity, the 

 pressure to which the heart, arteries, veins, and other intra- 

 thoracic organs are subjected, is considerably less than that of 

 the atmosphere. What is required to measure the difference 

 is to connect one pleural cavity with a manometer. This is 

 easily effected in the following manner : A glass tube of about 

 three millimetres in diameter is sealed at one end, and drawn 

 out to a blunt point. A hole is then cut with a sharp three- 

 cornered file on one side of the tube, close to the sealed end, 

 and the open end temporarily closed with a plug of wax. A 

 rabbit having been secured on the rabbit support, the skin is 

 perforated with a scalpel close to the left edge of the middle 

 of the sternum. This having been done, the point of the tube 

 is easily passed into the right pleura by pushing it in a hori- 

 zontal direction behind the sternum, with its point against the 

 posterior (i. e., as the animal is placed, the under) surface of 

 the thoracic wall. The wax plug is then removed, and the 

 open end is connected with a water manometer; but while this 

 is being done, great care must be taken to keep the side of the 

 tube on which the orifice is, firml}' but gently applied against 

 the chest wall. The quantity of water in the manometer is 

 then increased or diminished until the two columns stand at 

 the same level. If now the tube is twisted round so that its 

 orifice looks towards the cavity of the chest, the distal column 

 sinks, the difference between the heights of the two columns 

 in millimetres being about thirteen times as great as the dif- 

 ference in millimetres of mercury between the atmospheric 

 pressure and that to which the thoracic organs are subjected 

 in the animal under observation. The intra-thoracic pressure 

 may also be measured indirectly immediately after death, by 

 connecting the trachea air-tight with a manometer, and then, 

 after seeing that the two columns stand at the same level, 

 opening both pleural cavities. This time the distal column 

 rises above the proximal. The difference between them, if the 

 same kind of animal is used, will be the same, though in the 

 opposite direction. If it is desired to obtain a record of the 

 variations of intra-thoracic pressure during the respiratory 

 acts, it is easily done by bringing the tube into communica- 

 tion with a Marey's tympanum, by means of a somewhat thick- 

 walled India-rubber connector. In this way the tracing, fig. 

 249, is obtained. 



