GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 205 



resulting impulses being blocked on their way to the heart it 

 goes on beating regularly. After stimulation of the nerve 

 has been continued for several minutes the cooled tract of the 

 nerve is allowed to warm again until it becomes capable of 

 transmitting a nervous impulse; then the heart-beat is found 

 to be promptly stopped or slowed. This shows that if the 

 cardiac endings of the nerve be protected from fatigue, pro- 

 longed stimulation of the nerve-trunk does not interfere with 

 its functional capacity: the stimulation still starts nervous 

 impulses in it, which as soon as they can pass on produce their 

 normal effect on the heart. When long-continued sensations 

 become dulled the explanation is no doubt similar: it is the 

 end-organs, central or peripheral, or both, which are ex- 

 hausted, not the nerve-fibres themselves. It has, however, 

 been observed that when artificial stimulation is long applied 

 to one point on a nerve-trunk that point sometimes becomes 

 unexcitable, though the nerve in general is still quite func- 

 tional and acts perfectly when the point of application of the 

 stimulus is shifted a little: this is especially the case with 

 gray nerve-fibres and white fibres having a thin medullary 

 sheath. 



The very sparse blood-supply of nerve-trunks is in great 

 contrast to the rich supply of those parts of the nervous system 

 containing nerve-cells and to the abundant supply of muscles, 

 aiid is an evidence that the chemical changes taking place in 

 them during both rest and activity are but small. Seeing 

 that functional activity leads to little or no using up of the 

 conductive substance of a nerve-fibre any more than the 

 transmission of a galvanic current uses up a copper wire, the 

 term irritable is not properly applicable to nerve-fibres. Ir- 

 ritability in its physiological sense we have defined as a con- 

 dition of a living tissue such that a very small extraneous 

 force acting on it may cause it to set free a disproportionately 

 large amount of energy, and in that sense muscle-fibres and 

 nerve-cells are truly irritable, and they both use up their ma- 

 terial when at work and are subject to exhaustion. Nerve- 

 fibres are excitable and conductive, but not really irritable, 

 though on account of their great excitability they are very 

 generally spoken of as irritable. 



The Rate of Transmission of a Nervous Impulse. 

 This can be measured in several ways. One of the simplest 

 is a modification of the simple nerve-muscle experiment il- 



