234 VETERINARY PHYSIOLOGY 



Pulmonary Valve.- -The posterior cusp is mounted on the top 

 of the septum ventricuH, and is at a somewhat lower level 

 than the other two. 



Thus, in each valve, the cusp placed lowest is mounted on 

 a muscular cushion, the use of which will afterwards be con- 

 sidered. 



Attachments and Relations of the Heart (fig. 111). In the 

 horse the heart hangs downwards from the vertebral column, 

 and the apex is in relation to the posterior end of the sternum 

 and a little to the left. 



Behind the heart is in relation to the tendon of the 

 diaphragm. 



All round it are the lungs, completely filling up the rest of 

 the thorax. 



The heart is enclosed in a strong fibrous bag, the Pericardium, 

 which supports it and prevents over-distension. When fluid 

 accumulates in this bag the auricles are pressed upon and the 

 flow of blood into them is impeded. 



B. Physiology of the Heart 

 The Cardiac Cycle 



Each part of the heart undergoes contractions and relaxa- 

 tions at regular rhythmical intervals, and the sequence of 

 events from the occurrences of any one event to its recurrence 

 constitutes the cardiac cycle. 



A. Frog. In the frog a contraction, starting from the 

 openings of the veins, suddenly involves the sinus venosus, 

 causing it to become smaller and paler. This contraction is 

 rapid and of short duration, and is followed by a relaxation, 

 the cavity again regaining its former size and colour. As 

 this relaxation begins, the two auricles are suddenly contracted 

 and pulled downwards towards the ventricle, at the same time 

 becoming paler, while the ventricle becomes more distended 

 and of a deeper red. The rapid brief auricular contraction 

 now gives place to relaxation, and, just as this begins, the 

 ventricle is seen to become smaller and paler, and, if held in 

 the finger, is felt . to become firmer. This event takes place 

 more slowly than the contraction of either sinus or auricles. 



