CHAPTER XVIII 



STRUCTURE AND ACTION OF THE MAMMALIAN HEART 



THE heart of a sheep or man should have been previously dissected to show 

 its cavities and the blood-vessels connected with them, as well as the arrangement 

 and action of the auriculo-ventricular and semilunar valves. 1 A dissection 

 should also have been made in a rabbit of the nerves accompanying the carotid 

 artery (vagus, sympathetic, depressor). 



Action of the heart in man. Observe the chest wall over the 

 situation of the heart ; notice and feel the impulse or apex beat, 

 strongest at one spot ; mark this with ink. Apply the ear directly or 

 through a binaural stethoscope over this spot, and also over the second 

 right costo-sternal articulation Whilst listening to the sounds of the 

 heart feel the carotid pulse of the subject, and determine that the 

 first sound is systolic -i.e., is synchronous with the rise of pressure 

 in the artery due to the contraction of the ventricle. 



Cardiographie tracing. Apply the button of a cardiograph (fig. 63) 

 to the point where the impulse is most distinct, and take a tracing 

 upon a moderately fast drum by the aid of a recording tambour. 



The breath should be held whilst the tracing is taken, to eliminate the 

 movements caused by respiration. To obtain a distinct record it is best to select 

 a thin subject, and to place him on a couch in the recumbent position on his 

 left side. 



Rate of rhythm of the heart. Effect of position. Count the rate 

 of the heart's beat by placing the finger either over the apex beat or 

 upon an artery (pulse). Do this with the subject (1) recumbent, 

 (2) sitting up, (3) standing up. Note down any differences you may 

 observe in the rate and also in the character of the beat in these 

 different postures. 



Observation o the heart of a mammal in situ. This observation may be made 

 upon a " Sherrington " preparation. The animal (cat) has been killed under 

 chloroform anaesthesia by decapitation, the carotids having first been ligatured, 

 and the vertebrals occluded by a wire passed immediately in front of the axis 

 vertebra, drawn tightly and securely fastened behind. Another ligature is 



1 The mode of dissecting a sheep's heart is described in Pembrey's Practical 

 Physiology and in Stewart's Physiology. 



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