THE PIGEON. 199 



one another so as to include an obtuse dihedral angle, the 

 transverse sternal angle. 



80. The middle xiphoid process, a narrow, flat, 

 median plate, forming the hinder end of the corpus 

 sterni. 



81. The internal xiphoid processes, flat paired 

 plates proceeding outwards and backwards from the corpus 

 sterni at the point of origin of the middle xiphoid process, 

 with the posterior border of which they are connected by 

 narrow bridges of bone : two large fenestrae, covered in the 

 recent state by membrane, are thus enclosed between the 

 middle and internal xiphoid processes. 



82. The external xiphoid processes, similar to but 

 much larger than the internal xiphoids, and passing back- 

 wards and outwards from the lateral borders of the corpus 

 sterni, about half way between its anterior end and the 

 origin of the internal xiphoids. 



83. The costal border, or that part of the outer edge 

 of the sternum lying anterior to the external xiphoid process : 

 it is produced in front into a forwardly directed costal 

 process, and presents four facets for the articulation of the 

 sternal ribs. 



84. The manubrium or rostrum sterni, a small blunt 

 median prolongation of the anterior border of the corpus 

 sterni. 



85. The pneumatic aperture, a small aperture on the 

 dorsal surface of the corpus sterni about a centimetre behind 

 its anterior border : through it the interclavicular air-sac 

 ( 164, p. 218) communicates with air spaces in the bone : 

 these spaces can be seen as a delicate transparent network 

 by holding up the sternum to the light. 



86. The coracoid grooves, deep paired fossae on the 

 anterior border of the body of the sternum, extending 



