CIRCULATION OF THE BLOOD. 55 



aorta sufficient to support a column of blood nine feet in height ; that is, 

 must exert energy sufficient to raise tf Ib. 9 feet, or # X 9 or 2 X ll)S - one 

 foot. Work done is estimated by the amount of energy required to raise 

 a definite weight a definite height, the unit, the foot pound, being that 

 required to raise one pound one foot. 



The heart, therefore, at each systole exerts energy sufficient to raise 3 foot 

 pounds, and as it contracts 72 times per minute, it would raise in that 

 time 3 X 7 2 or 2l6 foot pounds; and in one hour 216 X 6o or 12,960 foot 

 pounds; and in 24 hours 12,960 X 2 4 or 3 II >4 foot 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 opening 

 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 Pneumogastric 

 (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- 



