ADJUSTMENT THE NERVOUS SYSTEM 131 



it may be merely reduced in intensity. In the latter case, when it 

 reaches a normal place again, it returns to its original strength, 

 since it requires just as severe a treatment to abolish it completely 

 as the normal impulse does. Such behaviour reminds one rather 

 of that of a train of gunpowder which is very narrow in one part. 

 If set alight* at one end, the evolution of energy decreases as the 

 chemical reaction passes along the narrow part, but it recovers 

 again to its original value when it arrives at the part of the same 

 width as the initial part, whereas a physical change, such as a 

 sound wave, does not recover its original intensity after having 

 been diminished by passing through cotton wool. On the whole, 

 it cannot be said that the nature of the nerve impulse is yet solved. 



The Visceral Nervous System 



Those organs and tissues composed of smooth or involuntary 

 muscle, such as the contractile coats of the intestines, heart, blood 

 vessels, and so on, receive a nervous supply which differs in several 

 ways from that of voluntary muscle. In the first place, as we have 

 seen, there are both excitatory and inhibitory nerves ; and, in the 

 second place, these nerves are in reality the axons of association 

 neurones and belong to the central nervous system, since they form 

 synapses with a further set of neurones outside the nervous system, 

 sometimes situated in the organ supplied, sometimes in masses of 

 nervous tissue, ganglia, distinct from these organs. It is the axons 

 of these neurones that pass to the actual tissue cells. In this 

 system are also included fibres which go to secretory glands as 

 well as to muscle. 



A definite set of these visceral fibres is known as the sympathetic 

 nervous system, and arises from a limited region in the middle part 

 of the spinal cord. Some of these fibres supply smooth muscle, 

 others glands, but all of them have the remarkable property of 

 being set into activity by the secretion of two ductless glands at 

 the upper ends of the kidneys, the supra-renals or adrenals. The 

 agent responsible for this effect is known as adrenaline, and has 

 been separated in the pure state. 



Although the viscera have sensory nerves also, it should be 

 noted that these nerves are similar in their nature and anatomical 

 relations to the ordinary sensory nerves, so that the involuntary 

 nervous system of the special nature described above is efferent 

 only. 



