2. The Pericardium ([p. 279]). In a specimen the thoracic contents of which have not been injured, dissect the mediastinal septum from the pericardial sac and remove fat about the great blood-vessels so as to expose them all fully. Study their relations. Prick and inflate the pericardium. Slit it lengthwise over the ventricles and reflect it so as to expose its contents. Study the attachment of serous and fibrous layers to the heart and their relation to one another.
3. In dissecting the heart follow the course of the blood, studying each cavity with the aid of the descriptions (p[p. 275]-279) as you proceed. Dissect as follows:
a. Remove the dorsal wall of the right auricle ([p. 275]) and of its appendage except that part of it to which the venæ cavæ are attached.
b. Introduce the probe from the right auricle into the right ventricle, and feel with the probe the line along which the ventricular wall joins the septum. Cut along this line so as to turn back the ventricular wall as a flap, which remains attached at the base of the ventricle.
c. Introduce a probe through the conus arteriosus into the pulmonary artery and cut along the probe.
d. Remove the dorsal wall of the left auricle and its appendage, but do not remove that part to which the pulmonary veins are attached.
e. Make a longitudinal incision beginning at the apex and divaricate the lips of the cut as you pass toward the base, thus avoiding injury to the lateral flap of the bicuspid valve. Without injury to the flap or the columnæ carneæ, remove the heart-wall at the sides of this incision near the base of the ventricle, as much as necessary to expose the cavity.
f. Pass a probe into the aorta. Introduce scissors behind the septal flap of the bicuspid valve and slit the aorta without injury to the bicuspid valve.
II. Dissection of the Blood-vessels.
Preparation and Injection.—A new specimen must be prepared for the dissection of the blood-vessels. The same specimen may be used, if necessary, for the dissection of the peripheral nervous system,—one side being used for the blood-vessels, the other for the nerves,—but it is much better to use separate specimens for the two systems.