THE CIRCULATION 131 



ing tambour. Another convenient form consists of a long rubber tube about 2 

 cm. in diameter and closed at one end. Its other end is connected with a recording 

 tambour by means of small tubing (Fig. 84). The interior of the large tube is occu- 

 pied by a long spiral spring. When applied transversely around the chest this tube 

 will be lengthened on inspiration and air will be drawn into it, causing the membrane 

 of the recording tambour to be displaced inward. On expiration, the recoil of the 

 tube, aided by the spiral, increases the pressure within its lumen and forces the 

 rubber membrane of the tambour outward. Marey's stethograph (Fig. 85) consists 

 of a metal plate which is fastened to the surface of the chest by means of a strap. 

 The changes in the tension of this plate suffered by it in consequence of the respira- 

 tory movements, are communicated by means of a lever to a receiving drum. The 

 latter, in turn, communicates with a recording drum. 



