THE CIRCULATION OF THE BLOOD. 123 



orifices of the aorta and pulmonary artery. The influence of the valves 

 in producing the sound is illustrated by the experiment performed on 

 large animals, such as calves, in which the results could be fully appre- 

 ciated. In thesa experiments two delicate curved needles were inserted, 

 one into the aorta, and another into the pulmonary artery, below the line 

 of attachment of the semilunar valves, and, after being carried upwards 

 about half an inch, were brought out again through the coats of the 

 respective vessels, so that in each vessel one valve was included between 

 the arterial walls and the wire. Upon applying the stethoscope to the 

 vessels, after such an operation, the second sound had ceased to be audi- 

 ble. Disease of these valves, when so extensive as to interfere with their 

 efficient action, also often demonstrates the same fact by modifying or 

 destroying the distinctness of the second sound. 



One reason for the second sound being a clearer and sharper one than 

 the first may be, that the semilunar valves are not covered in by the thick 

 layer of fibres composing the walls of the heart to such an extent as are 

 the auricula-ventricular. It might be expected therefore that their 

 vibration would be more easily heard through a stethoscope applied to 

 the walls of the chest. 



The contraction of the auricles which takes place in the end of the 

 pause is inaudible outside the chest, but may be heard, when the heart 

 is exposed and the stethoscope placed on it, as a slight sound preceding 

 and continued into the louder sound of the ventricular contraction. 



The Impulse of the Heart. 



At the commencement of each ventricular contraction, the heart may 

 be felt to beat with a slight shock or impulse against the walls of the 

 chest. The force of the impulse, and the extent to which it may be 

 perceived beyond this point, vary considerably in different individuals, 

 and in the same individual under different circumstances. It is felt 

 more distinctly, and over a larger extent of surface, in emaciated than in 

 fat and robust persons, and more during a forced expiration than in a 

 deep inspiration; for, in the one case, the intervention of a thick layer 

 of fat or muscle between the heart and the surface of the chest, and in 

 the other the inflation of the portion of lung which overlaps the heart, 

 prevents the impulse from being fully transmitted to the surface. An 

 excited action of the heart, and especially a hypertrophied condition of 

 the ventricles, will increase the impulse; while a depressed condition, or 

 an atrophied state of the ventricular walls, will diminish it. 



Cause of the Impulse. During the period which precedes the ven- 

 tricular systole, the apex of the heart is situated upon the diaphragm 

 and against the chest-wall in the fifth intercostal space. When the ven- 

 tricles contract, their walls become hard and tense, since to expel their 



