CHAPTER XVIII 

 Structure and Action of the Mammalian Heart 



Dissection. The human heart or that of a sheep 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. The auriculo-ventricular bundle 

 should also be shown. 1 A dissection 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 ; the second sound 

 which immediately follows being diastolic. 



Cardiographic tracing. Apply the button of a cardiograph (Fig. 66) 

 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 with a slow pulse, 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 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 of 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 



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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