THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS 189 



In the mediastinum in the median line ventral to the anterior part of the heart 

 and extending forward will be found a mass of gland tissue, the thymus. It is 

 larger the younger the specimen. In searching for it do not injure the large 

 blood vessels occurring in this region. The thymus is possibly one of the glands 

 of internal secretion and is derived from the entodermal lining of certain of 

 the visceral pouches of the embryo. 



Now press the heart and the left lung over to the right. The lung will be 

 found attached by a narrow region, the radix or root of the lung. An artery, a 

 vein, and a bronchus or air tube pass to the lung and veins from the lung in the 

 root, but these structures are better investigated at a later time. In the cat, 

 furthermore, the lung is attached along most of its length to the dorsal thoracic 

 wall by the pulmonary ligament, a fold of the pleura. Note that dorsal to the root 

 of the lung the pleura continues onto the dorsal and lateral surfaces of the 

 pleural cavity and that certain structures can be seen that lie internal to the 

 pleura. These structures lie between the dorsal portions of the two walls of 

 the mediastinal septum and consequently are situated in the mediastinum. 

 The most conspicuous of these structures lying in the mediastinum is the dorsal 

 aorta, a very large vessel injected with a colored solution which arches away from 

 the heart to the left and descends toward the diaphragm. About one-half an 

 inch ventral to the aorta is another tube, the esophagus, also lying in the medias- 

 tinum. Trace it posteriorly to the place where it penetrates the diaphragm. 



The diaphragm is a curved sheet forming the posterior wall of the thoracic 

 cavity and completely separating it from the abdominal cavity. The center of 

 the diaphragm is seen to consist of connective tissue forming a circular tendon, 

 the central tendon of the diaphragm. The remainder of the diaphragm is muscu- 

 lar. The diaphragm takes origin from the ribs, sternum, and vertebrae, and 

 is inserted on the central tendon. It is an important respiratory muscle. When 

 contracted, it flattens, thus lengthening the pleural cavities posteriorly and caus- 

 ing air to rush into the lungs. The diaphragm is pierced at several points to 

 allow important structures to pass through; the chief ones which penetrate the 

 diaphragm were already noted, i.e., the esophagus, the aorta, and the postcaval 

 vein. The diaphragm is a structure peculiar to mammals. It is formed in part 

 of the transverse septum and in part of other coelomic membranes; it then 

 becomes invaded by muscle buds from the adjacent cervical myotomes. 



Make a diagram of a cross-section through the thorax showing the pleural 

 and pericardial cavities and the relation of their linings to the thoracic wall, lungs, 

 and heart. 



4. The peritoneal cavity and its contents. Make a longitudinal slit through 

 the abdominal wall, a little to the left of the median ventral line from the inguinal 

 region up to the diaphragm. Widen the opening by a transverse slit in the 

 middle of the left abdominal wall. A large cavity, the abdominal or peritoneal 



