THE CIRCULATION 295 



The sympathetic exerts its augmentor effect very slowly, but it affects 

 both ventricle and auricle (Gaskell). 



THE INFLUENCE OF THE VAGUS on the heart is in general opposite 

 to that of the sympathetic. The fine medullated efferent fibers of the 

 vagus-trunk arise in the floor of the fourth ventricle of the brain in a 

 cluster of nerve-cells near the tip of the calamus scriptorius not far then 

 from the respiratory centers with which they are obviously closely con- 

 nected functionally. The course of the fibers between this center and 

 the heart is still in doubt. It is, for example, not certainly known as 

 yet what relations, if any, they have with the spinal accessory. It is 

 apparent that the (preganglionic) fibers passing to the heart-muscle end 

 in ganglion-cells situated mostly in the auricles, and that (postganglionic) 

 fibers of the unmedullated sort pass from these cells, one perhaps from 

 each to the muscle-tissue Here again there is difference in the experi- 

 mental product of various researches, and the anatomy of the vagus 

 from brain to heart-cells is obviously very incomplete. 



The action of the vagus nerve on the heart is in the direction of a 

 lessening of that organ's activity. In other words, it is inhibitory. 

 Stimulation of the vagus produces inhibition in several respects (Gaskell). 

 It lengthens the diastole of the heart and thus decreases the number 

 of beats in a given time. It lessens the force of systole, making the 

 auricles and ventricles contract less vigorously. It diminishes the ton us 

 (size, etc.) of the organ. It decreases the conductivity of the contractile 

 impulse to the muscle. It changes the electrical state of the heart- 

 muscle, making it more positive. 



The whole subject of inhibition is a mysterious one, but its great 

 importance as a mode of animal functioning becomes continually clearer. 

 In the heart one sees an excellent example of it, but how is it brought 

 about? GaskelPs trophic theory is at present receiving more notice 

 than any other hypothesis, and many things of different kinds point to 

 its probable truth. This supposition is, in a word, that inhibition in the 

 heart means a balance of the anabolic and the katabolic processes in the 

 nutrition of its protoplasm. Action implies katabolism in the active 

 cells, but an increase in constructive, anabolic process would tend to 

 check these destructive katabolic changes, to rest the heart, and to 

 increase its store of energy. In the frog's heart one sees in the secondary 

 augmentation noted below evidence that the organ's energy rises during 

 the period of inhibition, and in mammals the same tendency obtains. 

 The anabolic (vagal) influence seems to interfere more with the kata- 

 bolism than does the katabolic (sympathetic) influence with the anabolism 

 of the muscle. Until more than is now known is learned about the 

 metabolic processes of muscle in general little can be done to prove or 

 to disprove this interesting theory. It appears at any rate to be a step 

 toward the solution of the problem of trophic influence which the nervous 

 system is supposed to exert, and it may be useful later on in helping out 

 our knowledge of secretion. Just now it is the easiest way to explain (?) 

 the inhibition of the heart and, by analogy, of the other viscera. 



