170 JAY ARTHUR MYERS 



extremities are very narrow. This arrangement gives a very- 

 strong wall to this portion of the trachea, and because of this 

 specialization this portion is known as the tympanum. The 

 most caudal, or first, tracheal ring is smaller than the other 

 three and is transitional in size and shape between the tracheal 

 rings immediately above it and the intermediate syringeal car- 

 tilages immediately below it. As a whole, the tympanum arches 

 distinctly cephalad, its ventral and dorsal extremities projecting 

 caudad. 



For the proper understanding of the arrangement of the 

 remaining cartilages a description of the pessulus (fig. 3) here 

 becomes necessary. In size, the pessulus far exceeds all other 

 skeletal parts. It is a well developed bar of bone located at the 

 junction of the bronchi, and lying dorso-ventrally in a plane 

 transverse to the long axis of the trachea. Its ventral and dorsal 

 extremities are large and serve for the attachment of some of the 

 cartilages referred to above. 



The bone as a whole may be described as consisting of a shaft 

 and two large extremities (fig. 3). The shaft corresponds to a 

 little more than the middle third of the bone and in shape re- 

 sembles a three-sided prism with rounded borders. The cephalic 

 porder projects into the lumen of the trachea, and marks the 

 point of its bifurcation. The lateral surfaces of the shaft form 

 the medial walls of the cephalic ends of the bronchi, between the 

 diverging courses of which the basal surface lies. 



The ventral extremity is very large and may be considered as 

 being a pyi-amid whose apex points cephalad. On this pyramid 

 three distinct borders, a dorsal and two lateral; three surfaces, a 

 ventral and two dorso-lateral ; and a base, may be described. 

 The dorsal border is continuous with the cephalic border of the 

 shaft and projects into the lumen of the trachea. The two 

 lateral borders give attachment to the intermediate syringeal 

 cartilages which will be described presently. The ventral or 

 anterior surface is broad and sUghtly convex and stands out so 

 prominently that when the syrinx is viewed as a whole, this sur- 

 face projects farther ventrad than any immediately neighboring 

 skeletal element. The caudal portions of the dorso-lateral sur- 



