NERVES OF THE HEART. CARDIAC MUSCLE. 271 



the sympathetic, taking somewhat different paths in different 

 animals. In the frog their course is shown in Fig. 99; in 

 mammals most of them come from the upper thoracic gan- 

 glion of the sympathetic and the neighboring parts of the 

 main sympathetic chain. If the heart of a frog be exposed 

 and watched while the branch s, Fig. 99, is stimulated its 

 beat is seen to be quickened, especially if the previous rate 

 were slow: and quite similar phenomena may be observed 

 when the corresponding nerves are stimulated in a rabbit or 

 dog. And the beat is not merely made more rapid: it is dis- 

 tinctly more powerful for the time, the heart driving out more 

 blood at each stroke (even though pressure in the aorta may 

 be high) and thus doing increased work. 



Though the augmentor fibres reach the heart through the 

 sympathetic they have their centre (cardio-accelerator centre) 

 in the medulla oblongata, from which in mammalia they pass 

 down the spinal cord to the anterior roots of the upper tho- 

 racic spinal nerves, to the communicating branches, to the 

 sympathetic ganglia, and thence to the cardiac plexus and 

 the heart. Their centre, like the inhibitory, may be reflexly 

 excited : powerful stimulation of a sensory nerve, after section 

 of the vagi, usually quickens the pulse if the accelerator fibres 

 passing from the thoracic ganglia be intact, but has no effect 

 if these be previously divided. If the vagi are not cut the 

 result is not so certain, as the afferent impulses may also 

 excite the cardio-inhibitory centre and cause a mixed action: 

 but speaking generally afferent impulses which in a conscious 

 animal would cause acute but not extreme pain cause increase 

 of the heart- beat. This by raising general arterial tension 

 would for the time put the animal in good condition to make 

 a vigorous effort, and so is obviously an unconscious adaptation 

 of the organism for the preservation of its safety. While ex- 

 treme pain or extensive injury involving many afferent nerves 

 tends to cause fainting and loss of consciousness, the cardio- 

 inhibitory centre getting the upper hand. 



The Influence of Temperature Changes and of Calcium 

 Salts on the Heart-beat. If the excised heart of a frog be 

 cooled it beats more slowly; if heated, more quickly; until 

 the temperature approaches the limit at which muscle passes 

 into rigor. The observation is more difficult with mammals, 

 but if the heart of a dog be completely separated from all 

 the rest of the body except the lungs and supplied with blood 



