RIBS 



69 



posterior ones arise entirely from the transverse processes, which 

 increase in size correspondingly. Eight or nine ribs reach the 

 sternum, and from the eighteenth vertebra backwards the trans- 

 verse processes no longer bear ribs, but only short cartilaginous 

 apophyses. 



Flat, curved cartilages, or uncinates, are present in connection 

 with the ribs in the Crocodilia as well as in Hatteria. 



Birds. The ribs of Birds exhibit a much more marked 

 segmentation into vertebral and sternal portions, both of which 

 become ossified, and this evidently stands in relation to their 

 more active respiration. Bony uncinatcs, comparable to those 

 mentioned above, arise from, and usually become ankylosed to, 

 the vertebral portions in nearly all Birds, and overlap the ribs 

 next behind them (Fig. 53). The whole costal apparatus is 

 usually rendered still firmer by the 

 fusion of many of the trunk vertebrae 

 (p. 60), by the individual ribs often 

 being very broad, as well as by the 

 form and arrangement of the sternum 

 and pectoral arch. The last three or 

 four cervical vertebrae may bear com- 

 paratively large and movable ribs. 

 The number of ribs which articulate 

 with the sternum varies between two 

 (Dinornis elephantopus) and nine 

 (Cygnus). The delicate ribs of Archse- 

 opteryx (Fig. 49) more nearly resembled 

 those of Lizards.. 



Mammals. The cervical ribs in 

 nearly all cases unite completely with 

 the vertebrae, and a vertebrarterial 

 canal is thus formed. The last cervical 

 rib may be well developed and may 

 articulate with the corresponding 

 vertebra (e.g. Cholcepus hofmanni). 1 The seventh cervical rib 

 is also long in Bradypus, and the eighth and ninth ribs do 

 not reach the sternum ; they may therefore be counted as 

 cervical. There is considerable variation with regard to the 

 number of ribs which reach the sternum (e.g. in Manatus 

 2-3, in Cebus and Ateles 10) : and in some cases the sternal, 

 as well as the vertebral ribs may become ossified. In the 

 vertebral portion a capitulum, a neck, and a tubercuhim may be 

 distinguished (Fig. 54). The capitulum usually articulates 

 with its own centrum as well as with that next in front, 

 in the region of the epiphysis ; the tuberculum articulates with 



1 As amongst Reptiles, the ventral cartilaginous portions of some of the 

 anterior " false ribs " are connected with those in front, while the posterior ribs 

 end freely in the body- wall ("costaj fluctuantes "). 



FIG. 54. COSTAL ARCH OF 

 MAN. 



(7a, capitulum ; (7o, neck, Cp, 

 bony vertebral, and Kn, 

 cartilaginous sternal por- 

 tion of rib ; Ps, neural 

 spine ; Pt, transverse 

 process; St, sternum ; Tb, 

 tuberculum ; WK, centrum 

 of vertebra. 



