128 HAND-BOOK OF PHYSIOLOGY. 



vical ganglion. Unlike the inhibitory fibres of the pneumogastric, the 

 accelerating fibres are not continuously in action. 



The accelerator nerves must not, however, be considered as direct 

 antagonists of the vagus; for if at the moment of their maximum stimu- 

 lation, the vagus be stimulated with minimum currents, inhibition is 

 produced with the same readiness as if these were not acting. 



The connection of the heart with other organs by means of the nerv- 

 ous system, and the influences to which it is subject through them, are 

 shown in a striking manner by the phenomena of disease. The influence 

 of mental shock in arresting or modifying the action of the heart, the 

 slow pulsation which accompanies compression of the brain, the irregu- 

 larities and palpitations caused by dyspepsia or hysteria, are good evidence 

 of the connection of the heart with other organs through the nervous 

 system. 



The action of the heart is no doubt also very materially affected by 

 the nutrition of its walls by a sufficient supply of healthy blood sent to 

 them, and it is not unlikely that the apparently contradictory effect of 

 poisons may be explained by supposing that the influence of some of them 

 is either partially or entirely directed to the muscular tissue itself, and 

 not to the nervous apparatus alone. As will be explained presently, the 

 heart exercises a considerable influence upon the condition of the pressure 

 of blood within the arteries, but in its turn the blood-pressure within the 

 arteries reacts upon the heart, and has a distinct effect upon its contrac- 

 tions, increasing by its increase, and vice versd, the force of the cardiac 

 beat, although the frequency is diminished as the blood-pressure rises. 

 The quantity (and quality?) of the blood contained in each chamber, too, 

 has an influence upon its systole, and within normal limits the larger the 

 quantity the stronger the contraction. Kapidity of systole does not of 

 necessity indicate strength, as two weak contractions often do no more 

 work than one strong and prolonged. In order that the heart may do its 

 maximum work, it must be allowed free space to act; for if obstructed, 

 in its action by mechanical outside pressure, as by an excess of fluid within 

 the pericardium, such as is produced by inflammation, or by an over- 

 loaded stomach, or what not, the pulsations become irregular and feeble. 



THE AKTEKIES. 



Distribution. The arterial system begins at the left ventricle in a 

 single large trunk, the aorta, which almost immediately after its origin 

 gives off in its course in the thorax three large branches for the supply 

 of the head, neck, and upper extremities; it then traverses the thorax 

 and abdomen, giving off branches, some large and some small, for the 

 supply of the various organs and tissues it passes on its way. In the 

 abdomen it divides into two chief branches, for the supply of the lower 



