40 A MANUAL OF VETERINARY PHYSIOLOGY 



Third Period. — The aortic and pulmonary valves close, the 

 second sound of the heart is produced, followed by a long pause, 

 during which diastole of both auricles and ventricles occurs, the 

 auriculo-ventricular valves open, and blood flows into all the 

 chambers. 



The impulse of the heart, to which we have previously referred 

 as being felt externally between the fifth and sixth ribs, is not 

 given by the apex, but by the lower half of the left ventricle. 

 There is no such thing as an apex-beat ; the apex practically does 

 not move as long as the heart is retained within the pericardium, 

 but if the latter be opened, the apex is tilted forward with each 

 contraction. The Use of the Pericardium is to prevent over- 

 distension of the heart. 



Cardiac Sounds. — There are four sources of sound in the heart, 

 but as they work in pairs only two sounds are heard. We have 

 previously indicated where these occur in the heart's cycle. The 

 first sound is a long, booming one, and is made up of two causes — 

 the muscle sound of the contracting ventricle and the closure of 

 the auriculo-ventricular valves. The proof that the valves are 

 not wholly responsible for the first sound is that the bloodless, 

 beating heart still gives out a sound during ventricular systole. 



The cause of the second sound has never been doubted ; it is 

 due to the closure of the aortic and pulmonary valves ; it may 

 be abolished by hooking these back, and re-established by releas- 

 ing them. Under pathological conditions when the aortic valve 

 is destroyed a murmur takes the place of the normal heart sound. 

 When an animal is bled to death the second sound disappears 

 before the first — in fact, it is abolished immediately the amount 

 of blood propelled into the aorta is insufficient to distend this 

 vessel properly. 



As the first sound of the heart is heard just before the systole of 

 the ventricle, it is termed the ' systolic ' ; while the second sound, 

 occurring at the beginning of diastole, is termed the 'diastolic.' 

 On auscultation the two sounds are heard with unequal intensity 

 at different parts of the cardiac area. They are heard better on 

 the left side than the right, not because the heart is nearer to 

 that side than to the other, but for the reason that there is a 

 larger gap in the left lung, which exposes the heart and allows 

 its impulse to be felt against the chest wall. The two sounds 

 are very accurately represented by the words tub diip. 



Intracardiac Pressure. — Most important additions to the 

 physiology of the heart have been made by studying the 

 pressure existing in its chambers. The pressure exerted upon 

 the blood by the heart varies from moment to moment ; the 

 pump is for ever being charged and discharged, and both these 

 processes depend upon the condition of internal pressure existing 



