A "HEAD OF PRESSURE" 115 



way, for after all the nerve paths to the heart were cut, 

 Professor Starling found that the heart still responded if 

 increased work were thrown on it. The greater the 

 volume of blood that entered it, the more did its main 

 chamber expand and the stronger grew the force of its 

 stroke. There is a degree of filling and expansion of 

 the main chamber at which the heart-pump works best. 

 Now, when the muscles of the body are set in action 

 their movements force blood along the veins towards 

 the heart, thus filling it more quickly and to a greater 

 extent than before. In this way muscles can set the heart 

 at work. The musculature of the heart at once responds 

 to the increased load of blood just as an overladen horse 

 does when it has to breast a hill. The very words 1 

 use to explain this remarkable faculty of response which 

 is inherent in the heart show how little we understand its 

 mechanism. 



It is when we turn to the stopcock contrivance that 

 we get most light on the machinery by which the muscles 

 of the body control the work and output of the heart. 

 In fig. 30 a drawing is given of a vascular stopcock-; 

 superficially it has no resemblance at all to the form 

 which we are familiar with in water-taps, yet it serves 

 the same purpose as stopcocks which regulate the flow 

 in water-mains. A small artery is seen ending in a 

 microscopic part of the capillary field (fig. 30) ; the artery 

 has muscular fibres set as circles or spirals in its wall ; 

 they are minute spindle-shaped fibres which can be made 

 to contract and thus constrict, even close, the artery ; they 

 can also be made to relax, and thus permit the artery to 

 dilate and allow blood to flow more freely. These cuff's 

 of muscular fibres represent stopcocks ; they regulate 

 the outflow of blood from the . arterial compression 

 chamber of the cardiac pump. The higher centres of the 

 brain have no control over them ; they are entirely 

 managed by nerve centres in the spinal cord. As these 

 terminal arteries number tens of thousands, and each of 

 them is regulated and controlled, one can perceive how 

 complex the stopcock system of the human machine is. 



