THORACIC VISCERA OF SOME LARGE MAMMAL 
IOI 
see the three semilunar valves which guard the artery. Lay the 
artery open by a longitudinal slit passing into the ventricle and 
study the valves more carefully. Draw. 
Similarly dissect and study the semilunar valves of the aorta. 
Draw a diagram showing the physiological anatomy of the heart 
and large blood vessels , and indicate by arrows the course of the blood 
through them. 
D. PHYSIOLOGICAL DEMONSTRATIONS. 
1. The Contraction of the Heart in the Frog. 
The animal used for this purpose should be either pithed or 
etherized. Note that a complete cardiac cycle consists of (i) a 
wave of contraction (systole) which proceeds from auricle to 
ventricle and drives the blood on, emptying each part successively, 
and (2) a period of relaxation (diastole), during which the auricles 
refill from the veins, and the ventricle (the frog heart has only one) 
refills from the auricle. 
2. The Sounds of the Beating Heart in the Liviug Human Subject; 
By means of the phonendoscope (or some other form of 
stethoscope) or by placing the ear directly in contact with the 
chest over the region of the heart, listen carefully to the sounds 
and learn to distinguish them. The first sound is due to the 
ventricular systole and is attributable to a combination of causes 
among which the vibration of the contracting muscle is undoubt¬ 
edly the chief one. The second sound, which is the shorter 
and the sharper one, is caused by the sudden and simultaneous 
closure of the semilunar valves which guard the entrances to the 
pulmonary artery and the aorta, respectively. Determine the 
rate of heart beat when the subject is sitting, standing, and after 
vigorous exercise Record. 
