1. In the following directions " dorsal" means the side of the heart naturally turned towards the vertebral column, "ventral" the side next the breast-bone; "right" and "left" refer to the proper right and left of the heart when in its natural position in the body; " anterior" means more towards the head in the natural position of the parts; and " posterior" the part turned away from the head.

2. Get your butcher to obtain for you a sheep's heart, not cut out of the bag (pericardium), and still connected with the lungs. Impress upon him that no hole must be punctured in the heart, such as is usually made when a slaughtered sheep is cut up for market.

3. Place the heart and lungs on their dorsal sides on a table in their normal relative positions, and with the windpipe directed away from you. Note the loose bag (pericardium) in which the heart lies, and the piece of midriff (diaphragm) which usually is found attached to its posterior end.

4. Carefully dissecting away adherent fat, etc., trace the vessels below named until they enter the pericardium. Be very careful not to cut the veins, which, being thin, collapse when empty, and may be easily overlooked until injured. As each vein is found stuff it With raw cotton, which makes its dissection much easier.

a. The vena cava inferior; find it on the under (abdominal) side of the diaphragm; thence follow it until it enters the pericardium, about three inches further up; to follow it in this part of its course, turn the right lung towards your left and the heart towards your right.

The vein just below the diaphragm may be seen to receive several large vessels, the hepatic veins.

As it passes through the midriff, two veins from that organ enter it Between diaphragm and pericardium the inferior cava receives no branch; but, lying on its left side, will be seen the lower end of the right phrenic nerve, ending below in several branches to the diaphragm.

b. Superior vena cava: seek its lower end, entering the pericardium about one inch above the entry of the inferior cava; thence trace it up to the point where it has been cut across; stuff and clean it.

e. Between the ends of the two venae caves will be seen the two right pulmonary veins, proceeding from the lung and entering the pericardium; clean and stuff them.

5. Turn the right lung and the heart back into their natural positions; clear away the loose fat in front of the pericardium, and seek and clean the following vessels in the mass of tissue lying anterior to the heart, and on the ventral side of the windpipe.

a. The aorta: immediately on leaving the pericardium this vessel gives off a large branch; it then arches back and runs down behind the heart and lungs, giving off several branches on its way.

b. The pulmonary artery : this will be found imbedded in fat on the dorsal side of the aorta. After a course, outside the pericardium, of about an inch, it ends by dividing into two large branches (right and left pulmonary arteries), which subdivide into smaller vessels as they enter the lungs.

c. Observe the thickness and firmness of the arterial walls as compared with those of the veins; they stand out without being stuffed.

6. Notice, on the ventral side of the left pulmonary artery, the left pulmonary veins passing from the lung into the pericardium.

7. Up to this point the dissection may be made before the meeting of the class; on the preparation demonstrate the anatomical facts above noted and then proceed as follows:

8. Slit open the pericardiac bag, and note its smooth, moist, glistening inner surface, and the similar character of the outer surface of the heart. Cut away the pericardium carefully from the entrances of the various vessels which you have already traced to it. As this is done, you will notice that inside the pericardium the pulmonary artery lies on the ventral side of the aorta.

9. Note the general form of the heart—that of a cone with its apex turned towards the diaphragm. Very carefully dissect out the entry of the pulmonary veins into the heart. It will probably seem as if the right pulmonary veins and the inferior cava opened into the same portion of the organ, but it will be found subsequently (13. a.) that such is not really the case. Note on the exterior of the organ the following points: a. Its upper flabby auricular portion into which the veins open, and its denser lower ventricular part.

b. Running around the top of the ventricles is a band of fat, an offshoot of which runs obliquely down the front of the heart, passing to the right of its apex, and indicating externally the position of the internal partition or septum which separates the right ventricle, which does not reach the apex of the heart, from the left, which does.

a. Note the fleshy "auricular appendages"—one (left) appearing below the pulmonary artery; the other (right), between the aorta and superior cava.

10. Dissect away very carefully the collection of fat around the origins of the great arterial trunks and that around the base of the ventricles. In the fat will be found— a. A coronary artery arising from the aorta close to the heart, opposite the right border of the pulmonary artery; it gives off a branch which runs in the groove between right auricle and ventricle, and then runs down the dorsal side of the heart on the ventricles.

b. The other coronary artery, considerably larger, arises from the aorta dorsal to the pulmonary artery; its main branch runs along the ventral edge of the ventricular septum.

e. The coronary veins and sinus: small coronary veins will be seen accompanying the arteries; for the coronary sinus see 11. c.

11. Open the right ventricle by passing the blade of a scalpel through the heart about an inch from the upper border of the ventricle, and on the right of the band of fat marking externally the limits of the ventricles, and noted above (9.b.), and then cut down towards the apex, keeping on the right of this line; cut off the pulmonary artery about an inch above its origin from the heart, and open the right auricle by cutting a bit out of its wall, to the left of the entrances of the venae cavœ. On raising up by its point the wedge-shaped flap cut from the wall of the ventricle, the cavity of the latter will be exposed.

a. Pass the handle of a scalpel from the ventricle into the auricle; and also from the ventricle into the pulmonary artery, and make out thoroughly the relations of these openings.

b. Slit open the auricular appendage; note the fleshy projections (columnæ carneæ) on its walls, and the smoothness of the rest of the interior of the auricle. Observe the apertures of the vena cavœ, and note that the pulmonary veins do not open into this auricle.

c. Behind or below the entrance of the inferior cava, note the entrance of the coronary sinus; pass a probe through the aperture along the sinus and slit it open; notice the muscular layer covering it in.

12. Raise up by its apex the flap cut out of the ventricular wall, and if necessary prolong the cuts more towards the base of the ventricle until the divisions of the tricuspid valve come into view.

a. Note the columnæ carneæ on the wall of the ventricle, and the muscular cord (not found in the human heart) stretching across its cavity. Also the prolongation of the ventricular cavity towards the aperture of the pulmonary artery.

b. Cut away the right auricle, and examine carefully the tricuspid valve, composed of three membranous flexible flaps, thinning away towards their free edges; proceeding from near these edges are strong tendinous cords (chordae tendineae), which are attached at their other ends to muscular elevations (papillary muscles) of the wall of the ventricle.

c. Slit up the right ventricle until the origin of the pulmonary artery comes into view. Looking carefully for the flaps of the semilunar valves, prolong your cut between two of them so as to open the bit of pulmonary artery still attached to the heart. Spread out the artery and examine the valves.

d. Each flap makes, with the wall of the artery, a pouch, opposite which the arterial wall is slightly dilated. The free edge of the valve is turned from the heart, and has in its middle a little nodule (corpus Arantii).

13. Open the left ventricle in a manner similar to that employed for the right. Then open the left auricle by cutting a bit out of its wall above the appendage. Cut the aorta oft about half an inch above its origin from the heart. The aperture between left auricle and left ventricle can now be examined; also the passage from the ventricle into the aorta, and the entry of the pulmonary veins into the auricle; and the septum between the auricles and that between the ventricles.

a. Pass the handle of a scalpel from the ventricle into the auricle; another from the ventricle into the aorta; and pass also probes into the points of entrance of the pulmonary veins. Observe that no other veins open into this auricle.

b. Slit open the auricular appendage; note the fleshy projections (columnæ carneæ) on its interior, and the general smoothness of the rest of the inner wall of the auricle. Notice the columnæ carneæ over the inner surface of the ventricular wall, also the considerable thickness of the latter, as compared with that of the right ventricle or of either of the auricles.

e. Carefully raise the wedge-shaped flap of the left ventricle, and cut on towards the base of the heart, until the valve (mitral) between auricle and ventricle is brought into view; one of its two flaps will be seen to lie between the auriculo-ventricular opening and the origin of the aorta.

Examine in these flaps their texture, the chordae tendineae, the columnæ carneæ, etc., as in the case of the right side of the heart (12).

d. Examine the semilunar valves at the exit of the aorta; then cutting up carefully between two of them, examine the bit of aorta still left attached to the heart, and note the valves more carefully as described in 12. d. Note the origins of the coronary arteries in two of the three dilatations (sinuses of Valsalva) of the aortic wall above the semilunar flaps.

14. Examine a piece of aorta. Note that when empty it does not collapse; the thickness of its wall; its extensibility in all directions; its elasticity.

15. Compare with the artery the thin-walled flabby veins which open into the heart.