THE CIRCULATION 239 



for forty-eight hours. When the heart has come to a pause, 

 it cannot be started again by stimulating any nerve. It has 

 in the most marked degree its own views as to the rapidity 

 and force with which it ought to beat. But within certain 

 limits it is under nervous control. The accelerators hasten it, 

 to its own detriment. They belong to the division of katabolic 

 nerves a name given them to indicate that they waste the 

 tissues, impoverishing their condition. The vagus nerve slows 

 the heart. It protects it from itself. Its action is anabolic. 

 The condition of the heart is improved under its influence. 

 If it has been kept in check for a time by stimulation of the 

 vagus, the heart beats more strongly when this nerve ceases to 

 act than it did before it was induced to rest. 



The arteries also are under the influence of two antagonistic 

 sets of nerves. Those which increase their tonic contraction 

 are almost universal in their distribution. It may be that 

 those which actively check it are equally widespread, but 

 the evidence is not altogether free from ambiguity. On 

 certain organs such as the salivary glands, already instanced 

 which require great variations in the amount of blood supplied 

 to them, the influence of dilator nerves is very marked. The 

 simplest hypothesis as to the mode of action of vaso-constrictor 

 and vaso-dilator nerves leaves the initiative with the muscle- 

 fibres of the vessel- wall. The distending internal pressure of 

 blood is the stimulus which induces the muscle to contract. 

 In some invertebrate animals the snail, for example if 

 blood be prevented from entering the heart, so that there is 

 no distending pressure, the heart stops. In higher animals 

 the heart has acquired a habit of contracting, which keeps it 

 going in the absence of its proper stimulus. The two classes 

 of nerves exercise opposing influences on the muscle. Vaso- 

 constrictor nerves increase the excitability of its fibres ; vaso- 

 dilator nerves diminish it. Only thus can we explain their 

 action on a common basis. A good deal might be said as to 

 the reasonableness of such an explanation. Our views as to 

 the relation of nerve-influence and muscle-contraction are apt 

 to go astray, owing to the fact that generations of physiologists 

 have observed the phenomenon of a spasm of a muscle following 

 on a sudden stimulus to a nerve. The two events are evidently 

 related. The stimulus appears to set up a new condition in 



