248 THE HUMAN BODY. 



each, burner are required which regulate the gas-supply to 

 each flame independently of the rest. A similar arrange- 

 ment is found in the Body. Certain nerves control the 

 calibre of the arteries supplying different organs and, when 

 the latter are set at work, allow their arteries to dilate and 

 so increase the amount of blood flowing through them 

 while the general circulation elsewhere remains practically 

 unaffected. The resting parts at any moment thus get 

 just enough blood to maintain their healthy nutrition and 

 the working parts get more; and as certain organs come to 

 rest and others are set in activity, the arteries of the one 

 narrow and of the others dilate ; in this way the distri- 

 bution of the blood in the Body is undergoing constant 

 changes, parts which at one time contain much blood at 

 another having but little. In addition, then, to nervous 

 organs regulathrg the work of the heart and the arteries 

 with reference to one another, we have to consider another 

 set of vascular nerves which govern the local blood-supply 

 of different regions of the Body. 



The Nerves of the Heart. The heart gets nerves from 

 three sources. (1) From nerve-cells buried in its own sub- 

 stance and known as its intrinsic ganglia. (2) From the 

 tenth pair (pneumogastrics) of cranial nerves. (3) From 

 the sympathetic nervous system. The intrinsic ganglia 

 keep the heart beating, and the other two sets of nerves 

 control the rate and force of the beat. 



The Intrinsic Heart-Nerves. The ganglia of the heart 

 lie for the most part in the partition between the auricles 

 and along the line of junction of the auricles and ventricles ; 

 a few are found also in the upper parts of the latter. From 

 some of them arise nerve-fibres which go to the muscles of 

 the heart, while others are connected with the endings of 

 the extrinsic nerves reaching the organ ; and probably all 

 communicate by a network of nerve-fibres. 



The heart is an automatic organ : its beat, like the 

 movement of filaments of a ciliated cell, depends on its 

 own structure and properties and not on anything outside 

 itself. This is proved by the fact that the heart cut out of 

 an animal which has just been decapitated, and entirely re- 



