APPENDIX. 231 



4. Next pick up with forceps the remaining tissues of the ventral 

 wall near its posterior end, and carefully divide them longitudinally 

 a rittle on the left side of the middle line; being very careful not to 

 injure either the viscera in the cavity beneath or a large vein (ante- 

 rior abdominal) running along the wall in the middle line. 



5. About the point where you see this vein passing from the wall 

 to enter among the viscera of the ventral cavity, youwill come to the 

 bony and cartilaginous tissues of the sternal region. Raise the 

 posterior cartilage in your forceps, make a short transverse cut in 

 front of the vein, and, looking beneath the sternum, note the peri- 

 cardium with the heart beating inside it. Divide the fibrous bands 

 which pass from the pericardium to the sternum, and with scissors 

 cut away sternum, etc., taking great care not to injure the heart. 



6. Push a rod about half an inch in diameter down tlie animal's 

 throat so as to stretch the parts, and then picking up the pericardium 

 in a pair of forceps, open it and gently cut it awa>y from about the 

 heart; push aside any lobes of the liver which lie on the latter organ. 

 In the heart thus exposed note 



a. Its beat; a regularly alternating contraction (systole) and dila- 

 tation (diastole). 



b. In consequence of the destruction of the spinal cord compara- 

 tively little blood now flows through the heart, but during the con 

 traction you will be able to observe that the ventricular portion, 

 which will be readily recognized, becomes paler; and during dias- 

 tole again becomes deeply colored, getting more or less filled up with 

 blood which shows through its walls. 



c. Observe that each contraction starts at the auricular end and 

 travels towards the ventricular; this may be more easily seen by- 

 and-by, when the heart begins to beat more slowly. 



7. The specimen may be put aside under a bell-jar with a wet 

 sponge, or a piece of flannel soaked in water. If kept from drying 

 the heart will go on beating for hours. 



8. To demonstrate the action of the valves of the heart, obtain two 

 uninjured sheep's hearts from a butcher. Remove them from the 

 pericardium, taking care not to injure the vessels. 



9. Cut off the apex of one heart so as to open the ventricles. Then 

 fill up the stumps of the aorta and the pulmonary artery with water. 

 As the water is poured in the semilunar valves will be seen to close 

 up and block the passage to the ventricle, so that the stump of the 

 vessel remains full for some time. The valves rarely act quite per- 

 fectly in a heart removed from the body and treated as above, but 

 they will support the water column quite long enough to illustrate 

 their action, 



