HEART 893 



Thus, in each valve the cusp placed lowest is mounted 

 on a muscular cushion, the use of which will afterwards be 

 considered. 



B. Attachments and Relations of the Heart. 



The heart is attached, by the great vessels coming from 

 it, to the dorsal wall of the chest. 



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 (fig. 168). 



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, in disease, fluid accumulates in this bag the auricles 

 are pressed upon and the flow of blood into them is impeded. 



C. Physiology of the Heart. 

 I. The Cardiac Cycle. 



Each part of the heart undergoes contractions and 

 relaxations at regular rhythmical intervals, a7id the sequence 

 of events from the occurrence of any one event to its recur- 

 rence constitutes the cardiac cycle. 



A. Frog. 



In the frog (fig. 169) a contraction, starting from the 

 openings of the veins, suddenly iuvolves 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 tAvo auricles are suddenly 



