1888.] 



On the Vertebral Chain of Birds. 



481 



free dorsal; its vertebral rib has a perfect sternal piece, and thus 

 there are four complete cinctures to the thorax. The last developed rib 

 is feebler, and its sternal piece does not quite reach the sternum. 

 There is a dia-parapophysial facet for its feeble upper part ; it is a 

 cup nearly as large as the diapophysial facet in front of it, and the 

 cartilage lining the cup is extended downwards on a narrow convexity 

 of the transversely carinate outgrowth, and thus this rib, with a small 

 head, and a neck less than half the normal size, articulates by one 

 continuous facet belonging to both tuberculum and capitulum, and 

 entirely on the neural arch. 



The third vertebra in the general sacral series has a pair of ribs ; 

 these have lost their capitulum entirely ; they are mere rods, 6 mm. 

 long and 075 mm. thick, and are ankylosed by their inner twisted 

 end to the diapophyses. 



After these come three pairs of strong pre -iliac buttresses — gener- 

 alised masses, from which all trace of rudimentary ribs has gone — in 

 the old bird. Then come two vertebras with the bodies nearly devoid 

 of lower outgrowths; these are the true sacrals. These are followed 

 by the urosacrals, the first of which has strong rib-bars that buttress 

 the post-ilia, and that are ossified as distinct riblets, but are not seg- 

 mented off as distinct tracts of cartilage in the embryo. But in old 

 birds the buttresses of the second true sacral are not quite absorbed, 

 but remain as prickles, for the clearing away of unnecessary parts 

 goes on even after the bird is adult. This is only one among many 

 instances that could be adduced in which the transformation of the 

 skeleton is seen to be continued throughout life. In that transforma- 

 tion, from beginning to end, each individual bird repeats the story of 

 its birth in the past ages, and each individual bird seems to be striving 

 towards some goal, albeit in its present state, when adult, its struc- 

 ture is to the morphologist an absolutely perfect thing. 



In birds, as a rule, the true sacrals abort, or even suppress, the 

 pleuroid rudiments in the true sacrals ; four of these block-like 

 vertebras form the sacrum proper of the Swan ; two only in the 

 Cormorant.* 



* Professor Huxley (op. cit., p. 416), in this third character of Birds as distin- 

 guished from Keptiles, says that, " Although all birds possess a remarkably large 

 sacrum, the vertebrae, through the intervertebral foramina of which the roots of the 

 sacral plexus (and, consequently, of the great sciatic nerve) pass, are not provided 

 with expanded ribs abutting against the ilium externally, and against the bodies of 

 these vertebra by their inner ends." Those true sacrals are called ' lumbo-sacral ' by 

 Professor Mivart ('Zool. Soc. Trans.,' vol. 10, p. 345, Plate 61, fig. ]), whilst the 

 first two " uro-sacrals" are called "sacral." This is certainly an erroneous nomen- 

 clature. 



Professor Mivart speaks of his examination of the skeleton of P. bicristatus and 

 P. brasiliensis , as well as of P. earbo. His figure of the pelvis is probably one 

 of these, and not of P carbo ; it differs from the two old specimens of the common 



