50 HUMAN PHYSIOLOGY. 



pounds; and in 24 hours 12,960 X 2 4 r S 11 ^ f ot pounds or 138.5 

 foot tons. 



Influence of the Nervous System upon the Heart. When the 

 heart of a frog is removed from the body, it continues to beat for a variable 

 length of time, depending upon the nature of the conditions surrounding 

 it. The heart of warm-blooded animals continues to beat but for a very 

 short time. The cause of the continued pulsations of the frog heart is the 

 presence of nervous ganglia in its substance. These ganglia have not 

 been shown to exist in the mammalian heart, but there is reason to believe 

 that the nervous mechanism is fundamentally the same. 



The ganglia of the heart are three in number, one situated at the open- 

 ing of the inferior vena cava (the ganglion of Remak), a second situated 

 in the auriculo- ventricular septum (the ganglion of Bidder), and a third 

 situated in the inter-auricular septum (the ganglion of Ludwig). The first 

 two are motor in function and excite the pulsations of the heart ; the third 

 is inhibitory in function and retards the action of the heart. The actions 

 of these ganglia, though for the most part automatic, are modified by im- 

 pressions coming through nerves from the medulla oblongata. When the 

 inhibitory centre is stimulated by muscarin, the heart is arrested in diastole ; 

 when atropia is applied, the heart recommences to ^beat, because atropia 

 paralyzes the inhibitory centre. 



The nerves modifying the action of the heart are the Pneutnogastric 

 (Vagus) and the Accelerator nerves. 



The Pneumogastric nerve, after emerging from the medulla, receives 

 motor fibres from the spinal accessory nerve. It then passes downward, 

 giving off branches, some of which terminate in the inhibitory ganglion. 

 Stimulation of the vagus by increasing the activity of the inhibitory centre 

 arrests the heart in diastole with its cavities full of blood ; but as the stimu- 

 lation is only temporary, after a few seconds the heart recommences to 

 beat; at first the pulsations are weak and feeble, but soon regain their 

 original vigor. After the administration of atropia in sufficient doses to de- 

 stroy the termination of the pneumogastric, stimulation of its trunk has no 

 effect upon the heart. The inhibitory fibres in the vagus are constantly 

 in action, for division of the nerve on both sides is always followed by an 

 increase in the frequency of the heart's pulsations. 



The Accelerator fibres arise in the medulla, pass down the cord, emerge 

 in the cervical region, pass to the last cervical and first dorsal ganglia of 

 the sympathetic, and thence to the heart. Stimulation of these fibres 

 causes an increased frequency of the heart's pulsations, but they are 

 diminished in force. 



