432 A MANUAL OF VETERINARY PHYSIOLOGY 



electrical condition is termed diphasic variation ; its presence in muscle 

 is concurrent with a contraction, while in nerve it is identical in 

 point of time and rate with the transmission of a nervous impulse. 

 We have seen that the current of injury may be detected by the 

 rheoscopic frog ; the current of action may be detected in the same 

 way. If the nerve of a nerve-muscle preparation be laid on the 

 heart of a frog which is still active, the current of action liberated 

 from the heart by its contraction causes the muscle of the limb to 

 contract at each heart-beat. 



The string galvanometer shows that when a muscle contracts two 

 electrical waves pass over it, due to changes in potential. Electro- 

 meter records of the contraction of the heart muscle may be ob- 

 tained by placing leads either on the exposed heart, or leading off 

 through the limbs. Waller was the first to show how an electro- 

 cardiogram could be obtained in an intact animal at each beat of the 

 heart. If a dog be placed with one fore-paw in a basin containing a 

 solution of salt, and a diagonal hind-paw in another, and the two 

 basins connected with the galvanometer, at every beat of the heart 

 the electrical changes resulting from its contraction are conducted 

 through the body to the electrometer, and the minute movements of 

 the mercury rendered visible by a microscope. Contraction begins 

 at the base of the heart ; the base is therefore negative to the apex. 

 When the ventricles contract, the cardiogram shows that the apex is 

 then negative to the base ; the interpretation of the curve is not, how- 

 ever, so simple as might appear. 



Nature of Nerve Impulse. — The velocity of a motor impulse in 

 man has been ascertained to be about ioo metres (333 feet) 

 per second ; the velocity in sensory fibres is unknown. In the 

 frog the velocity of the action-current in motor nerves agrees 

 exactly with the rate at which a motor impulse travels in the 

 animal, and in discussing the nature of nerve-impulses it is 

 natural that the action current, or negative variation, should 

 be considered to be intimately connected with the transmission 

 of ordinary normal impulses. It is known that in the case of the 

 passage of impulses along the optic nerve, or of light falling 

 on the retina, these are accompanied by electrical changes. 

 Action-currents can be obtained in motor nerves on stimulation 

 of the cerebral centres governing limb movements ; currents 

 of action can also be demonstrated in the living muscles of the 

 limbs ; diphasic variation of the working heart can be recorded ; 

 and it seems generally accepted that the passage of an impulse 

 along a nerve is associated with an electrical change, though it 

 is necessary to be careful and to avoid considering this change and 

 a nerve impulse as identical. Hitherto it has been usual to 

 regard a nerve-fibre as not suffering from fatigue ; the end organ, 

 it was known, could be exhausted, but not so the nerve. This 

 view was arrived at in consequence of the non-existence of 

 chemical products arising from the passage of impulses, the 

 presence of which would suggest that conduction was a question 

 of chemical activity. The fact that the passage of impulses 



