376 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 50 



the jugular veins. Ventrally from the vertebral column, the right 

 and left cervical sacs are separated from each other by only a thin 

 partition. Anteriorly they extend to the distal end of the tenth 

 cervical vertebra, posteriorly as far as the lung-margin. The external 

 border clings closely to the musculature of the neck and curves in a 

 dorsal direction, ending with a small oval extension, the pars ovalis 

 (fig. 12, SC2), which reaches as far as the diverticulum subscapu- 

 lare. Baer (1896, p. 432) says concerning these sacs: "Von der 

 Lungenspitze entspringend, reichen sie ungefa.hr bis zur Mitte des 

 letzten Halswirbels." I cannot agree with this. Although I have 

 seen many variations in the length of the cervical sacs, I have never 

 observed them so short as that in the pigeon. In conclusion it 

 should be mentioned that the sacci cervicales communicate with the 

 saccus interclavicularis. This communication is effected by means 

 of the diverticula subscapularia, which are described below. 



The Diverticula 



While the cervical sacs themselves are simple, their diverticula are 

 complicated. The sacs and their diverticula pneumatize the verte- 

 bral column and its muscles. Since the diverticula of the cervical 

 sacs extend both into the neck and into the thorax, they may be 

 divided into two distinct groups or systems belonging to these two 

 regions, the cervical and the thoracic. 



The Cervical System 



From the proximal end of the pars ovalis (fig. 12, SC 2) of each 

 cervical sac a flattened canal arises which usually penetrates the 

 cervical muscles in the region of the nth cervical vertebra, and then 

 turns forward, becoming a long, almost straight canal, the canalis 

 intertransversarius (figs. 3, 4, 5, 7, n, and 12, CI 2). This canal is 

 usually connected with the sac by a second communication further 

 on. Together with the ascending vertebral artery, the vertebral 

 vein and the deep cervical sympathetic nerve, the canalis intertrans- 

 versarius passes forward through the foramina transversaria formed 

 by the heads and tubercles of the ribs. Beyond a point lying in the 

 region between the 5th and 7th cervical vertebrae this canal (figs. 

 n and 12, CI 1) is enlarged like a spindle between the successive 

 foramina transversaria, through which it passes ; further on, it be- 

 comes simply cylindrical and finally ends on the distal side of the 

 atlas which it pneumatizes (figs, n and 12, CI 3). 



Two kinds of diverticula are given off from this canal, the diver- 

 ticula supravertebralia (figs. 3, 4, 5, 7, 11, 12, DSPV) and the diver- 



