DOUBLE EEGISTRATIOX. 283 



a bag enclosed in a tube, it consists of a metal box, across the interior of which 

 a septum of strong india-rubber is stretched. ,One side of the box is filled with 

 soda sohttion and connected with the artery ; the other with air and connected 

 wifli the tympanum, to is a piece of lead-tubing for connecting the sphygmo- 

 scope X with the tube in the artery. 



B is a forked tube of German silver or brass for connecting the ai'tery with the 

 kymograph ion. a is a cannula, wliicli is inserted into an artery (see fig. 3a). 

 i is a small ring, soldered to it, by which it may be tied to the rings e on B, to 

 prevent it from slipping off. e and e are two rings soldered to B. By means of 

 ligatures passed througli these, B may be fastened to the skin or hair of the 

 animal, to prevent its being displaced by any sudden movement. Tlie oblique 

 limb of B may be connected to A d alone by a piece of india-rubber tubing, or to 

 both A d and A to at the same time by means of a Y glasj tube and india-rubber 

 tubing. Another piece of india-rubber tubing is attaclied to the straight limb d, 

 and closed by a clamp or clip. When a clot forms, the clip is taken off and 

 the clot removed, the tube washed out by a sti'eam of soda solution, and the 

 clip again replaced. 



c 12 the tracing-pen (see fig. 3e). It is stuck horizontally through a piece of 

 cork ; another small piece of glass tvibing, about three-fourths of an inch long, 

 closed at its upper end, and about one-twelfth of an inch in diameter, or just 

 wide enough to admit the end of the swimmer, is stuck vertically into the same 

 piece of cork.* 



pen {g), which registers the movements of the mercury on the 

 revolving cylinder. 



The disadvantage of the mercurial manometer is, that it does 

 not give the true form or extent of the variations of hlood-pres- 

 sure, the inertia of the mercury causing it to oscillate ahov(i 

 and below the true value. "VYe may, however, obtain the true 

 form of each oscillation along with the mean pressure by con- 

 necting the artery at the same time with the manometer and 

 one of jMarey's sphygmoscopes and levers (Fig. 133, x and y) by 

 means of a Y-tube, oiie end of which is connected to d and the 

 other to w. In order to obtain the mean pressure, we turn the 

 stopcock ie) till, on blowing through it, only a slow rise and fall 

 of the mercury can be produced, but no sudden oscillation. On 

 then connecting it with the artery the mercurial column shows 

 the mean pressure, while the sphygmoscopic lever registers each, 

 oscillation (c/. Brunton and Cash, Phil. Trans., vol. clxxxii (1891) 

 B, pp. 547-550 and 601-632). 



Besides this form there are various others on the same prin- 

 ciple, some of which have cylinders which wind off a con- 



* The instrument figured here was made by Sauerwald for Ton Bezold, but 

 as Yon Bezold did no!: require it on account of illness it was sold to the author 

 by Sauerwald in 1867- 



