NERVES OF THE HEART. CARDIAC MUSCLE. 269 



are the same as the cardio-inhibitory or are a distinct set is 

 still uncertain. In mammals, also, it is frequently noticeable 

 that vagus inhibition of the heart is followed by a period of. 

 unusually powerful pulsation. 



The Cardio-inhibitory Centre. This consists of nerve- 

 cells lying in the medulla oblongata and giving origin to the 

 cardio-inhibitory fibres. In some animals it seems to be nor- 

 mally always in a state of slight' activity, sending out feeble 

 impulses which exert a slighUfcheck on the rate of pulse. 

 This is the case in the dog, for in that animal division of 

 both pneumogastric nerves in the neck is followed by a 

 quicker heart-beat: in the rabbit, on the other hand, the 

 centre appears usually at rest, as section of the pneumogas- 

 trics in that animal has no effect on the pulse-rate. Whether 

 normally in action or not the centre can readily be excited, 

 especially by afferent impulses reaching it through abdominal 

 nerves. If the intestines of a frog (the brain of which in 

 front of the medulla oblongata has been entirely removed so 

 as to make consciousness impossible) be exposed and sharply 

 struck, the heart stops in diastole; but if both cardiac nerves 

 have been previously divided this result does not follow. 

 The stoppage is clearly then a reflex inhibition through the 

 cardio-inhibitory centre and nerves, and the afferent tract can 

 be readily traced. The afferent impulses from the intestine 

 pass through the mesenteric branches of the sympathetic, for 

 if these be cut no cardiac standstill follows the mechanical 

 stimulation of the intestine, although the vagi be intact. If 

 only the communicating branches from the sympathetic gan- 

 glia to the spinal cord be cut or only the anterior roots of the 

 corresponding spinal nerves, or only the spinal cord above the 

 place of entry of these roots, or only the medulla oblongata 

 destroyed, yet, in each case, the intestinal stimulation causes 

 no stoppage of the heart. When the standstill does result it 

 is therefore reflex, the afferent path being sensory nerve-end- 

 ings in intestine, mesenteric nerves, sympathetic ganglion, 

 communicating branches, anterior spinal roots, spinal cord 

 to centre in medulla; the efferent fibres are the inhibitory 

 in the vagus. The fainting which in man not infrequently 

 follows a severe blow on the pit of the stomach is due to 

 similar reflex excitation of the cardio-inhibitory centre: and 

 the fainting seen during severe pain and that which certain 

 odors cause in some persons are due to similar stimulation of 



