CHAP, iv.] THE VASCULAR MECHANISM. 317 



fourth thoracic spinal nerve, and which is therefore in reality the 

 fourth thoracic ganglion, and so on to the rest of the thoracic chain. 



Now when the several rami communicantes, or the anterior 

 roots, of the lower cervical and upper thoracic nerves are separately 

 stimulated it is found that augmentor effects make their appear- 

 ance with considerable constancy when the second and third 

 thoracic nerves are stimulated ; the effects are less constant with 

 the first and fourth thoracic nerves; sometimes some effect may 

 appear with the fifth thoracic nerve, but not with any other 

 thoracic nerves, or with any of the cervical nerves. 



We may therefore say that in the dog, augmentor impulses 

 leave the spinal cord by the anterior roots of the second and third, 

 to some extent the first and fourth, and possibly the fifth 

 thoracic nerves, travel by the several rami communicantes to the 

 stellate ganglion and pass thence to the cardiac plexuses and so to 

 the heart, by nerves from the stellate ganglion itself, or from the 

 annulus of Vieussens, or from the so-called lower cervical ganglion. 

 In the cat the path of the augmentor impulses is very similar, and 

 we may regard the statement just made as representing in a broad 

 way the path of these impulses in the mammal generally. They 

 leave the spinal cord by the upper thoracic nerves and pass to the 

 heart through the lower cervical and upper thoracic sympathetic 

 ganglia. 



The effect of stimulating these augmentor fibres is in some 

 cases to increase the rapidity of the rhythm. When the heart is 

 beating very slowly this acceleration may be very conspicuous, but 

 when the heart is beating quickly or even at what may be called 

 a normal rate the acceleration observed may be very slight. A 

 more constant and striking effect is the increase in the force of the 

 beat. W T hen tracings are taken of the movements of the auricles 

 and ventricles separately it is observed that in the case both of the 

 auricles and of the ventricles, the extent of the systole is increased ; 

 moreover it would seem also that both cavities undergo a larger 

 expansion, they are filled with a larger quantity of blood during 

 the diastole. This means that the output of the heart is increased 

 by the action of the augmentor nerves, and that such is the effect 

 may be directly shewn by the cardiometer. Moreover this increase 

 of the output may take place in spite of a concomitant rise of 

 arterial pressure, so that the effect of the action of the augmentor 

 nerves is distinctly to increase the work of the heart ; and this may 

 take place even though no marked acceleration occurs. 



In the mammal as in the case of the frog, when the augmentor 

 fibres are stimulated, some time elapses before the maximum effect 

 is witnessed and the influence of the stimulation may last some 

 considerable time after the stimulation has ceased. 



When records are taken of the behaviour of the heart during 

 the stimulation of afferent nerves, such as the sciatic or the 

 splanchnic, the records shew that the heart may behave very much 



