OSSEOUS SYSTEM OE AVES. 



29 



21 



Front view of pelvis, Partridge. 



further developed as compared with the body ; it approaches to a 

 triangular form, expanding posteriorly, 

 where the margin is entire, and convex. 

 The depth of the keel exceeds the entire 

 breadth of the sternum. The coracoid 

 depressions are deeply trochlear: the 

 manubrial process is small, and directed 

 upward ; the costal processes are also pre- 

 sent, but of small size : the costal border 

 is short. In these pre-eminently volant 

 Vertebrates, the breast-bone reaches the 

 maximum of developement. 



§ 126. Sacral Ferfe/^r^.- -In vertebrate 

 anatomy the term ' sacrum ' is applied to 

 the centrum and neural arch of the ver- 

 tebra, having its haimal arch complete, as 

 in the thorax, but with its ap])endage de- 

 veloped into a hind-limb (vol. i., figs. 101, 

 D and 114). If two or more vertebras coalesce beyond the thorax, 

 they are likewise said to form ^ a sacrum,' although but one may 

 be typically complete, and the rest support only stunted pleura- 

 pophyses. In all warm-blooded Vertebrates the sacrum, when 

 present, is so characterized, and confluence is carried to an 

 extreme in birds, converting a large proportion of the vertebral 

 column into a '^sacrum,' fig. 21, .9, «, c, which in the Ostrich may 

 include seventeen or more vertebras. Thirteen is the average 

 number in Natatores, twelve in Grallatores and Rasores, eleven in 

 Altrices or the higher birds of flight.' 



In analyzing this most complex of all compound bones, in a 

 young Ostrich ^, I find the centrum of the first sacral vertebra 

 distinct, although its neural arch and spine have coalesced with 

 those of the second vertebra and with the ilia. Traces of the 

 articulation between the centrum of the second and third sacral 

 vertebra remain : they are obliterated in the remaining vertebras, 

 and the bodies of all are cellular and permeated by air. 



The pleurapophysis of the first sacral retains its moveable 

 articulation to the par- and di-apophyses of its vertebra ; it is 

 long, slender, and terminates in a free point. That of the second 

 sacral vertebra is styliform, half the length of the preceding, and 

 terminates in a free point projecting downward and backward ; 

 its head and tubercle, free in the young bird, become confluent 



» See the Table in vii'. p. 273. 



2 Nos. 1835 and 1837, Osteol. Scries, Mus. Coll. Chir. 



