PRACTICAL DIRECTIONS. 449 
the left and thus lies in a pocket dorsocaudad of the heart and 
dorsad of the vena cava inferior. 
2. The pleura. Each pleura is a membrane like the peritoneum. 
It covers the diaphragm and the thoracic walls and is reflected onto 
the lungs. Hence it is divided into costal and pulmonary portions. 
Read the description (p. 217) and trace their connection. ' Each 
pleura forms a closed sac. 
3. The mediastinal septum or median vertical portion formed by 
the apposition of the medial walls of the two pleural sacs. It divides 
the thorax into halves. 
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). 
4, Dorsally (i.e., in the posterior mediastinum) the trachea (best 
seen craniad), the cesophagus (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 a noting its double 
character. 
Dissect the mediastinal septum from the thie so as to expose 
this organ. 
@, 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 dis- 
sected. 
J. Salivary glands (p. 223). 
1. Keep the mouth open by a cork between the teeth. Find the 
papilla upon which open the sublingual and submaxillary ducts (pp. 
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. 
2. Find the white ridge formed on the cheek by the parotid duct 
(p. 223). It is opposite the molar tooth, and at its cranial end the 
