56 M. R. CHASE AND S. W. RANSON 



these rami. After giving off the bronchial rami the vagus joins 

 with the nerve of the opposite side in the formation of the eso- 

 phageal plexus. From the esophageal plexus small twigs are 

 given off to supply the esophagus. These twigs remove almost 

 all of the remaining medullated fibers, so that the nerve as it 

 enters the abdomen is composed almost entirely of non-medul- 

 lated fibers. 



Figure 18 is a low-power drawing of the anterior half of the 

 esophageal plexus. The fascicles of this plexus have been dis- 

 sected off from the esophagus, drawn close together, and stained 

 with osmic acid. There are very few medullated fibers in these 

 fascicles. A high-power drawing of fascicle a is seen in figure 19. 

 The few medullated fibers seen here are widely separated and of 

 very small size. Pyridine-silver preparations show that these fas- 

 cicles are almost entirely formed of sohdly packed, fine, non- 

 medullated axons. In figure 16 one sees a high-power drawing 

 from one of these preparations. In this field there are six med- 

 ullated fibers which are ahnost lost among the great number of 

 non-medullated axons. 



As the nerve passes through the diaphragm it contains very 

 few medullated fibers (fig. 20). It will be noted that figures 17 

 and 20 were taken from preparations of the cat's vagus. The 

 thoracic vagus in the cat, then, has the same structure as that 

 in the dog. The two figures represent the same nerve, the one 

 taken above the level of the pulmonary rami and the other as 

 the nerve pierces the diaphragm. A comparison of these two 

 figures shows how great is the decrease in the number of medul- 

 lated fibers as the nerve passes through the thorax. Those which 

 have disappeared have either left the vagus through the pul- 

 monary rami and fine branches to the esophagus, or have lost 

 their sheaths in their downward course. 



Gaskell also noted this decrease in the number of medullated 

 fibers and concluded that the vagus as it pierced the diaphragm 

 was composed chiefly of non-medullated fibers. 



