1892.] Physiology and Pathology of the Mammalian Heart. 439 



tion by a very well-marked increase in the force and frequence 

 of the auricular and ventricular contractions. They may be vaso- 

 constrictors for the coronary vessels, although we give no proof of 

 this. 



There are nerve fibres which descend to the heart by the vago- 

 sympathetics, which, on excitation under certain conditions, increase 

 the force and frequence of beat of the auricles and ventricles, and 

 which may be vaso-dilators for the coronary vessels. 



Keflex excitation of the vagus produces results which are the 

 same as those of direct excitation of the nerve, and the curves are 

 more typical and satisfactory than those obtained on direct excitation 

 of the nerve. 



Excitation of a mixed nerve like the sciatic usually produces 

 effects on the heart similar in kind to those due to direct excitation 

 of the augmentors, but the phenomena are complicated by the greater 

 rise of the pressure in the systemic arteries. Sometimes the increase 

 in force of the ventricle more than counterbalances this increased re- 

 sistance to contraction, and the amount of residual blood in the left 

 ventricle is reduced ; in other cases the increase in force of the ven- 

 tricular contractions is not sufficient to counterbalance the increased 

 resistance, and the residual blood in the left ventricle is increased. 



In Section IX we show that excitation of the central end of a 

 mixed nerve like the sciatic or splanchnic usually affects both the 

 augmentor and vagus centres in the medulla, and that, in nearly all 

 cases, the augmentor centre is the more strongly excited of the two, 

 so that angmentor effects show themselves during the excitation, but 

 are succeeded by vagus action on ceasing to excite the nerve. In 

 many cases augmentor effects alone show themselves. When excited 

 reflexly the augmentor centre ceases to act earlier than the vagus; 

 the opposite, therefore, to what takes place with direct excitation. 

 In rare cases the excitation of the vagus centre may be stronger than 

 that of the augmentor from the first. Although, in the absence of 

 any augmentor action, the vagus does not reduce the force of the 

 ventricular systole, it does unmistakably have the power of in- 

 hibiting the strengthening influence which the augmentors exert upon 

 the ventricular contractions. 



In Section X, upon the part played by the vagus in the economy, 

 we show that vagus excitation relieves the heart of work and there- 

 fore of waste to as great an extent as is compatible with a continua- 

 tion of the circulation, and conclude that the vagus acts as a 

 protective nerve to the heart, reducing the work thrown upon that 

 organ when from fatigue or other cause such relief is required by it. 

 The presence of fibres in the sciatic and other mixed nerves which 

 cause reflex excitation of the vagus would seem to indicate that this 

 nerve may be used by other parts of the body to diminish the out- 



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