Experiment 201. _The dissection of a sheep’s heart._ Get a sheep’s heart with the lungs attached, as the position of the heart will be better understood. Let the lungs be laid upon a dish so that the heart is uppermost, with its apex turned toward the observer. The line of fat which extends from the upper and left side of the heart downwards and across towards the right side, indicates the division between the right and left ventricles. Examine the large vessels, and, by reference to the text and illustrations, make quite certain which are the _aorta_, the _pulmonary artery_, the _superior_ and _inferior venæ cavæ_, and the _pulmonary veins_. Tie variously colored yarns to the vessels, so that they may be distinguished when separated from the surrounding parts. Having separated the heart from the lungs, cut out a portion of the wall of the _right ventricle_ towards its lower part, so as to lay the cavity open. Gradually enlarge the opening until the _chordæ tendineæ_ and the flaps of the _tricuspid valve_ are seen. Continue to lay open the ventricle towards the pulmonary artery until the _semilunar valves_ come into view. The pulmonary artery may now be opened from above so as to display the upper surfaces of the semilunar valves. Remove part of the wall of the right auricle, and examine the right auriculo-ventricular opening. The heart may now be turned over, and the _left ventricle_ laid open in a similar manner. Notice that the mitral valve has only two flaps. The form of the valves is better seen if they are placed under water, and allowed to float out. Observe that the walls of the _left_ ventricle are much thicker than those of the _right_. Open the left auricle, and notice the entrance of the _pulmonary veins_, and the passage into the ventricle. The ventricular cavity should now be opened up as far as the aorta, and the semilunar valves examined. Cut open the aorta, and notice the form of the _semilunar valves_.