Praeticum III. The Thoracic Septum. 21 



a. Notwithstanding the identity of name there is no special relationship between the 

 a/.ygous vein and that lobe of the right lung. Moreover, nether of them is strictly 

 a/.ygous or mesal. 



b. These large veins may not be recognized at first because they are 

 collapsed and covered by pleura. Either of them is most easily demon- 

 strated by pressing on the other two and carrying the fingers toward the 

 heart. 



27. The Phrenic Nerve. On the lateral aspect of the precava and 

 postcava and the intervening portion of the auricle is a whitish cord, the 

 RIGHT PHRENIC NERVE ; it comes indirectly from the myel (spinal cord) 

 in the neck and is distributed to the daphragm ; see the lecture on Res- 

 piration. 



28. The Thymus. Cephalad of the heart is a pale, lobulated mass, 

 resembling a salivary gland ; this is the THYMUS BODY, larger in young 

 cats, but in old ones sometimes insignificant. With the butchers both the 

 thymus and the pancreas of the calf are sold as "sweet-breads." 



29. The Thoracic Septum. Lift the sternum slightly and note in 

 the interval between it and the heart and thymus a delicate, transparent 

 membrane, the THORACIC SEPTUM. If no undue force has been used in 

 preparing and opening the thorax it will form a continuous sheet, trav- 

 ersed by some vessels and nerves ; it will be more fully examined later, 

 and is looked at now lest it be ruptured in opening the left thorax. 



30. Opening the Left Thorax. Remove the lateral parietes of the 

 left thorax as directed for the right ( 13), taking especial care not to pull 

 upon the sternum or cut too close to it. Lift the sternum and note the 

 completeness yet transparency of the septum. It appears to be single, 

 but is really double, the conditions being as follows : Each side of the 

 thorax is lined by its own pleura, a closed sack. The thymus, heart and 

 other mesal organs lie between the apposed mesal sides of the two pleural 

 sacks ; but for a certain space near the sternum these apposed layers are 

 in contact and apparentlv constitute a single membrane. 



a. The independence of the right and left parts of the thoracic cavity provides that, 

 in accident or disease, either side and its contained lung may be affected with less inter- 

 ference with the other. 



31. The Left Lung. Note the incomplete separation of the cephalic 

 and intermediate lobes of the left lung, and that there is no left azygous 

 lobe ; the right may be seen through the pleura. 



32. The Pulmonary Veins. If the lobes of the lung are displaced 

 carefully laterad, at their root may sometimes be seen the PULMONARY 

 VEINS full of blood. 



33. Amputate the lobes and observe the following parts. Their 

 pleural covering may render their outlines indistinct. 



a. The LEFT PHRENIC NERVE, crossing the septum at the pocket for 

 the azygous lobe ; it is often bordered by a line of fat ; the cephalic part 

 of its course may be less easily traced. 



b. Near the diphragm, two cylinders, the ventral fleshy and pinkish, 

 the ESOPHAGUS, already seen in the right thorax ; the dorsal, the AORTA, 

 the great artery from the heart. The two following are not usually seen 

 distinctly upon an alcoholic specimen. 



c. The THORACIC DUCT, a corrugated tube. 



d. The LEFT SYMPATHIC (sympathetic) NERVE, with its GANGLIA, 

 the slio-ht enlargements on the heads of the ribs. 



