THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS 183 



anterior intermediate air sac may also be punctured. Against the dorsal wall 

 of the pleural cavity on each side will be found a reddish, spongy flattened organ, 

 the lung. The openings of some of the air sacs into the lungs will probably be 

 noted on some specimens. On cutting into the lung the organ will be found to 

 be solid, not hollow, as in the preceding animals. 



The cavity in which each lung is contained is, as already explained, a pleural 

 cavity. It is lined by a coleomic membrane, the pleura. As the lungs are flat- 

 tened against the dorsal wall of the pleural cavity, the pleura passes over their 

 ventral faces, leaving them outside, so to speak. The pleura, furthermore, 

 passes over the surface of the pericardial sac and lines the inner surface of the 

 body wall. 



7. The syrinx. — Examine the posterior part of the trachea. Two slender 

 muscles, the sternotracheal muscles, diverge from their insertion on the ventral 

 surface of the trachea to their origin on the sternum. These muscles should be 

 severed. The trachea disappears dorsal to the heart and the great blood vessels 

 which enter and leave the heart. These blood vessels must not be injured. 

 Loosen the trachea and pull it forward. The bifurcation of the trachea into the 

 two bronchi can then be seen dorsal to the heart. Cut across the bronchi with 

 a fine scissors and draw the trachea forward. At the point where the trachea 

 forks into the two bronchi an expanded chamber, the syrinx, is present. The 

 voice of birds issues from the syrinx, not from the larynx. Along each side of the 

 trachea extending from the point of insertion of the sternotracheal muscles to 

 the lateral walls of the syrinx is a muscle, the intrinsic syringeal muscle. The 

 walls of the syrinx are supported by the last tracheal rings and the first bronchial 

 half-rings. The last two tracheal rings are widely separated from each other 

 but are connected in the median ventral line by median processes. Make a slit 

 in the ventral wall of the syrinx and spread apart the cut edges. The cavity of 

 the syrinx is named the tympanum. In the dorsal wall of the tympanum a 

 slight vertical fold is present in the median dorsal line, extending forward from 

 the level of the bifurcation of the trachea. This fold is called the semilunar 

 membrane, and its vibrations are said to produce the voice. There are also large 

 thickenings in the lateral walls of the tympanum which may have some function 

 in the production of the voice. The sternotracheal and syringeal muscles doubt- 

 less aid by changing the size and shape of the tympanum. 



G. THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS OF A MAMMAL 



The following directions apply to both the rabbit and the cat. Whenever 

 the differences between the two animals warrant, a separate description of each 

 will be given; otherwise they will be described together. 



1. The mouth cavity and the pharynx. — 



a) The salivary glands: The salivary glands are masses of gland tissue which 

 are outgrowths of the lining of the mouth cavity; the stalk of the outgrowth 



