MULLER] THE AIR-SACS OF THE PIGEON 377 



ticula medullaria. The supravertebral diverticula surround the 

 intervertebral articulations and increase the relative mobility of the 

 vertebrae. These diverticula are given off from the anterior ends 

 of the portions (spindle-shaped enlargements) of the canal lying 

 between the successive transverse processes. They extend medially, 

 are club-shaped, and terminate before reaching the median plane. 

 They lie dorsally, behind the vertebras, and are largest in the middle 

 of the neck, where the relative mobility of the vertebras is greatest. 

 The medullary diverticula are given off from the cervical canal just 

 in front of the foramina transversaria. They consist of extraverte- 

 bral and intravertebral portions. The extravertebral portions are 

 small and simple vesicles. The intravertebral portions, which I 

 name diverticula supramedullaria (fig. 12, DSPM I ; figs. 11 and 12, 

 DSPM2), enter the medullary canal through the intervertebral 

 foramina, and extend dorsally from the spinal cord. Within the 

 medullary canal they widen out and impinge upon the corresponding 

 diverticula of the opposite side. They partly unite with these as well 

 as with the adjacent diverticula (in front and behind) of the same 

 side, to form a continuous canal, sickle-shaped in transverse section, 

 and lying above the medulla, the canalis supramedullaris (figs. 3, 4, 

 5, 7, and 12, MEA). The partial absorption of the walls of these 

 diverticula which leads to the formation of this canal, takes place 

 during the growth of the bird, and posteriorly, near the thorax, 

 where the canal is widest, is usually quite completed in middle-aged 

 birds. Anteriorly this absorption decreases as the medullary diver- 

 ticula become smaller, the completely formed supramedullary canal 

 usually extending no farther than the third or fourth cervical vertebra. 

 Anterior to that it is replaced by two rows of isolated diverticula (fig. 

 12). The posterior end of the supramedullary canal lies near the last 

 cervical vertebra. Occasionally it communicates here with the cor- 

 responding canal of the thoracic vertebrae. Very delicate extensions 

 of the canales intertransversarii pneumatize the cervical vertebrae. 

 They penetrate the bone radially, but are otherwise quite irregular. 



The Thoracic System 



The diverticula of the sacci cervicales in the thorax are arranged 

 like those in the neck, but are not so highly developed. These 

 thoracic diverticula pneumatize the thoracic vertebrae and form a 

 supramedullary canal. Extravertebral outgrowths are absent. 

 From the distal end of the pars ovalis of either side a ventrally 

 flattened tubule arises. This passes between the vertebral muscles 

 and through the intervertebral foramen in front of the first thoracic 



