PHYSIOLOGICAL 239 



region of "decrement", it immediately regains all its original power 

 on entering the healthy part of the fibre on the far side. In man 

 the impulse travels at a rate of about 300 to 400 feet per second ; 

 but in lower animals it may be much slower than this. 



The behaviour of the impulse in regaining its full strength on the 

 far side of a region of decrement led Keith Lucas to the analogy of a 

 train of gunpowder, along which an explosion advances. Now it has 

 been successfully demonstrated that in nerves, as in all living 

 tissues, a process of "burning" continually goes on; oxygen is used 

 up and carbon dioxide formed. Great difficulty has been found in 

 proving that the amount of carbon dioxide produced is increased 

 when the nerve is active, but Prof. G. H. Parker has been successful. 

 Nevertheless, many workers have concentrated their attention 

 rather on the electric currents ("currents of action") which appear 

 when an impulse travels along a nerve; and as there is much to 

 suggest that electrical forces are of primary importance in the 

 starting of the nerve impulse, and in its transmission from one fibre 

 to another, the chemical aspect of the nerve impulse has been some- 

 what neglected. Quite recently, however, A. V. Hill and his collabora- 

 tors have clearly shown by means of an ingenious apparatus of great 

 sensitiveness, that there is a definite production of heat by the 

 nerve fibre in the transmission of an impulse. The amount of heat 

 produced in one second's activity was about enough to raise the 

 temperature of the nerve by one ten- thousandth part of a degree. 

 If, as seems likely, this heat is produced by a "burning" process, it 

 corresponds to the formation of a quantity of carbon dioxide closely 

 approaching the quantity actually determined by Parker. It is plain 

 that some sort of chemical activity accompanies the transmission of 

 the impulse in nerve, but it is obvious that we are still far from 

 understanding one of the most familiar phenomena of life. 



REFLEXES 



When we sit resting and thinking, with one leg swinging freely over 

 the other at the knee, a sharp tap below the joint sends our dangling 

 foot up into the air. This is the familiar "knee-jerk reflex", often 

 used by physicians. What has happened when the rapid contraction 

 of the muscles of the lower leg sends our foot up with a jerk? The 

 mechanical stimulus of the knock has sent a thrill along sensory 

 nerve fibres to the spinal cord ; the impulse passes to connecting or 

 associative nerve cells in the cord, and thence to motor nerve cells, 

 whence, along motor nerve fibres, orders come to the muscles com- 

 manding them to contract. The brain is not required at all. A pre- 

 arranged linkage of sensory, associative, and motor spinal nerve 

 cells is sufficient in itself, with, of course, the muscle fibres, to give 



