244 



THE BODY AT WORK 



cutaneous constrictor nerves to let go their grip at the same time. 

 Terror heightens the control of the vaso-motor centre over the 

 vessels of the skin ; it increases Vagus inhibition of the heart. 

 Even disgust evoked by a revolting sight or a foul smell may 

 call 'the vagus so forcibly into action as to bring the heart to 

 a standstill. 



The Pulse. The arterial system is always distended. The 

 pressure in the largest arteries amounts to about 140 milli- 

 metres of mercury. The source of pressure is the beat of the 

 heart pushing the blood forward against the resistance offered 

 to its flow by the smallest vessels. At every stroke another 

 3 ounces is added to the already overfull vessels. In the aorta, 



FIG. 14. SPHYQMOGRAPH. 



A, An ivory button which is pressed on the skin over the radial artery by a metal spring. B, A 

 continuous screw which works against the cog-wheel C. By rotating B, the lever D is 

 raised to a position in which its point scratches the travelling-plate E (covered with black- 

 ened paper). F, A box containing clockwork which moves E. G, A screw by means of 

 which the pressure of the spring is adjusted to the force of the pulse. 



therefore, the blood moves forward with jerks, but by the time 

 it reaches the capillaries the intermittent accessions of force 

 have been taken up by the elastic walls of the vessels and 

 returned to the stream in the form of constant pressure. In 

 the very smallest arteries the blood flows in a steady stream. 

 If the corpuscles in a capillary vessel are watched under the 

 microscope, they show no variations in rapidity synchronous 

 with the beat of the heart. The " pulse " in the larger arteries 

 is the push given to the column of blood by the sudden con- 

 traction of the left ventricle. Its propagation along the 

 arteries will be understood if it is remembered that the blood is 

 contained within elastic tubes. The first effect of the ejection 

 into the aorta of an additional quantity of blood is the distension 

 of its wall. The wave of distension travels down all the 

 arteries of the body with gradually decreasing force. 



