238 



THE BODY AT WORK 



used with safety to lower blood- pressure. The discovery that 

 the inhalation of amyl nitrite answers this purpose and fulfils 

 this condition was the result. 



When we consider the hydrostatics of the circulation, it 

 becomes evident that changes in the force with which the 

 heart beats, and changes in the calibre of the bloodvessels, work 

 together in determining blood-pressure. Both vessels and 



FIQ. 13. MANOMETER FOR MEASURING BLOOD-PRESSTTRE. 



A U -tube -contains mercury, on which floats a rod supporting a scratching point, which makes 

 a " tracing " on blackened paper wrapped round a revolving drum. Between the mano- 

 meter and the cannula which is introduced into the central end of a cut artery is a three- 

 way cock, which leads to a pressure- bottle containing a half-saturated solution of sodic 

 sulphate. This solution prevents blood from clotting. Before it is connected with the 

 artery the apparatus is filled from the pressure-bottle. The cock is then turned into the 

 second position, and the bottle raised until the mercury in the manometer stands at a level 

 somewhat higher than that which it may be expected to attain under the influence of 

 blood-pressure. The cannula being then inserted into an artery, the cock is turned into 

 the third position, which places the manometer in connection with the blood, and excludes 

 the pressure-bottle. As the mercury is a little higher than blood-pressure, some of the 

 sodic sulphate solution enters the artery, but no blood enters the cannula. The scratchicg- 

 point, rising and falling with every variation in blood-pressure, makes a record on the soot- 

 blackened paper, which is subsequently removed from the drum, and varnished. 



heart contract automatically the former continuously, the 

 latter rhythmically. The heart of a frog, if it is enclosed in a 

 moist chamber, beats for a long time after its removal from 

 the animal. Even when cut in pieces, in certain ways, the 

 separate pieces beat. A strip from the ventricle of a tortoise's 

 heart, kept gently stretched by the weight of a light lever 

 attached to one of its ends, continued to contract rhythmically 



