4. In the mediastinum or space between the halves of the mediastinal septum are seen—
a. Ventrally (i.e., in the anterior mediastinum), blood-vessels and fat, and craniad the pink thymus gland. Read the description of the thymus ([p. 254], and [Fig. 107]).
b. Dorsally (i.e., in the posterior mediastinum) the trachea (best seen craniad), the œsophagus (best seen caudad), and the aorta.
c. In the middle (reckoning in a dorsoventral line) (i.e., in the middle mediastinum) the heart, with the inferior vena cava approaching it from the rear. Prick into the mediastinum and inflate it. Remove the thoracic wall opposite the one already removed, so as to stretch the mediastinum and show it more clearly. Expose the trachea beneath the caudal end of the sternomastoidei. Note the thyroid gland ([Fig. 96], 6, [p. 229]). Insert a blowpipe without injury to the thyroid, and inflate the lungs. Cut the sternum at its caudal end and turn it craniad, cutting the mediastinal septum and noting its double character.
Dissect the mediastinal septum from the thymus so as to expose this organ.
d. The pericardium, a relatively thick-walled sac, within which the heart lies. The pericardium is covered by the mediastinal septum on each side, but not dorsally and ventrally. Dissect the mediastinal septum from the pericardium. Prick and inflate the pericardium. Open it and expose the heart.
Dissection of the Alimentary Canal.
Study and verify the descriptions of the organs as they are dissected.
I. Salivary glands ([p. 223]).
1. Keep the mouth open by a cork between the teeth. Find the papillæ upon which open the sublingual and submaxillary ducts (p[p. 223] and 224); remove the mucosa and enlarge the opening and insert a black bristle into either duct. Trace the duct by the bristle as far as possible on the floor of the mouth.