PR/1CTICAL DIRECTIONS. 449 



the left and thus lies in a pocket dorsocaudad of the heart and 

 dorsad of the vena cava inferior. 



2. 'I'he pleurre. 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). 



d. Dorsally (i.e., in the posterior mediastinum) the trachea (best 

 seen craniad), the oesophagus (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, r>, 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 jtericardium 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. 



I. Salivary glands (p. 223). 



1. Keep the mouth open by a cork between the teeth. Find the 

 papdlae 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 



