THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS i8< 



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

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

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

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

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

 the visceral pouches of the embryo. 



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

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

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

 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 thoracis 

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

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

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

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

 the mediastinal septum and consequently are situated in the mediastinum 

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

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

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

 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 thoracii 

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

 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, anc 

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

 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 t< 

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

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

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

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

 becomes invaded by muscle buds from the adjacent cervical myotomes. 



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

 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 througl 

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

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

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



