TRANSPORT OF MATERIALS 149 



be supposed to rise, so that a mean pressure of 140 mm. is required 

 to keep up a flow of the same magnitude as before, as actually 

 takes place. The next heart beat will not be powerful enough to 

 raise the pressure above, say, 100 mm., but this will be done by the 

 expulsion of less blood than before, so that a certain amount is 

 left behind in the ventricle. Since the same amount as before flows 

 in from the veins, the ventricle starts the next beat at a greater 

 distension than that of the previous one, and, as we saw above, its 

 contractile energy is greater. The increase continues with each 

 beat until the original output is reached. 



Now, these facts and various others relating to the automatic 

 adjustment of the strength of the beat are readily and simply 

 explained by that property of muscle to which attention has already 

 been directed (p. 96). We saw that the amount of energy developed 

 in muscular activity is in direct proportion to the length of the 

 fibre during the time that it is in the act of developing its state of 

 tension. This energy, in fact, is proportional to the area of certain 

 surfaces arranged lengthwise in the fibre. A greater distension of 

 the cavities of the heart is necessarily accompanied by a stretching 

 of all the constituent muscle fibres of their walls, so that when 

 contraction takes place it starts from a greater length of the active 

 surfaces, and greater energy is produced. 



There is also what may be called an external control of the force 

 and rate of the heart beat. We have already seen that there are 

 two characteristics of smooth muscle in general, and the walls of 

 the heart behave as this kind of muscle, although they possess the 

 transverse striation of voluntary muscle. The first of these is that 

 of automatic activity, which may be manifested either in a state of 

 moderate contraction or by a series of rhythmical beats, or both 

 combined. This is very obvious in the case of the heart, which 

 continues to beat when cut out of the body, and will require 

 further consideration presently. The second characteristic is the 

 supply of two kinds of nerves, one increasing the state of activity 

 (excitatory}, the other decreasing it (inhibitory}. In the case of the 

 heart these functions are exercised both on the strength of the 

 beat and on its rate. The excitatory nerve fibres come from 

 the sympathetic system, and are known as the " augmentor " or 

 "accelerator" nerves. They may be looked upon as similar to 

 the vaso-constrictor nerves, which we found to come from the same 

 system. The inhibitory nerve fibres are contained in the vagus 

 nerves, and have a general depressant action, not only on the 

 strength and rate of the beat, but also on the excitability of the 

 muscle to stimuli and on its capacity to conduct the wave 

 of contraction. Both of these nerves can be set into action by 

 reflexes. The depressor nerve, which we saw to produce a fall of 



