SYMrxVTllETIC NERVES — CAllDIAC GANGLIA. 303 



the nerves, we divide the vagi and irritate them by an induc- 

 tion-coil, noting the strength of current required to produce 

 still-stand or slowness of the heart before and after injection of 

 the drug into the veins. 



Is the Sympathetic Paraly.^ed ? — This is tested by cutting the 

 vagi and dividing the spinal cord between the first and second 

 •cervical vertebrae, so as to exclude the action of tliose centres 

 in the head which quicken the heart and raise the blood-pressure ; 

 the drug is then injected, and the sympathetic irritated by an 

 induced current and the pulse counted. If it be quickened by 

 the irritation, the sympathetic is not paralysed. 



Are the Cardiac Ganglia Paralysed ? — To see whether or not 

 the nervous structures contained in the heart itself are acted on 

 by a drug, we must separate it from all other nerves passing to 

 it from without, and prevent its being acted on by anything 

 other than the drug, such as altered blood-pressure or tempera- 

 ture. This is done in mammals by dividing the vagi, the 

 sympathetic cord, the depressor, and the spinal cord between 

 the first and second cervical vertebrae. The heart is thus sepa- 

 rated from the quickening and retarding centres, so that any 

 iilteration in its beats must be due to the nerves contained in its 

 walls, or the muscular fibre of these walls themselves : at the 

 same time the vessels are separated from the vaso-motor centre, 

 and the heart is thus protected from the effects of any change 

 in the blood-pressure, except the generally unimportant ones 

 produced by the action of the drug on the vascular walls. The 

 number and amplitude of the heart's contractions are then 

 registered by a needle placed in the ventricle, and the blood- 

 pressure by the manometer ; poison is injected into the jugular, 

 and the tracings taken afterwards are compared with those 

 taken before. If we find that the heart-beats have become 

 slower and weaker, while the pressure they have to overcome 

 has not been increased, we may conclude that the motor nerves 

 or the muscular substance of the heart have become paralysed. 

 If the blood-pressure have risen, blood should be allowed to 

 iiow from an artery till it falls to its previous level, and then 

 tracings should be taken with the needle for comparison with 

 the previous ones. 



