188 SKELETON OF THE SWAN. 



groove on the fore part of the sternum. The clavicle (ib.), 

 58, articulates with the coracoid above, but is confluent 

 with its fellow and with the keel of the sternum below. 

 The iliac bones, 62, are remarkable for their length, and 

 for the number of the vertebrae, or the great extent of the 

 confluent spinal column, to which they are anchylosed. 

 They reach in the swan, and in most other birds, from 

 the tail forwards to the vertebrae with movable ribs. 

 Thus the artificial characters of a "lumbar vertebra" are 

 wanting. The pubis and ischium on each side have 

 coalesced with the ilium to form the lower boundary of 

 the widely-perforated acetabulum. The pubis is long and 

 slender, joins the ischium of its own side near its lower 

 extremity, but does not join its fellow ; thus the foramen 

 ovale is defined, but there is no symphysis pubis : the 

 absence of this symphysis facilitates the expulsion of the 

 large ovum with its unyielding calcareous shell. The 

 ischium coalesces posteriorly with the ilium, and converts 

 the ischiadic notch into a foramen. The caudal vertebra3, 

 Cdj are few in number, with broad transverse processes 

 formed by confluent pleurapophyses, the limits of which 

 may still be traced. A hasmapophysis is articulated to 

 the lower interspace, between the fourth and fifth caudal, 

 and is anchylosed to the sixth. The humerus of some of 

 the larger birds of flight — e. g. the pelican or adjutant 

 crane — is remarkable for its lightness, as compared with 

 its bulk and seeming solidity ; it is, in fact, a mere shell 

 of compact osseous tissue. The orifice admitting air to 

 its large cavity is beneath the great tuberosity at the 

 proximal end. 



The keel is excavated, not only for the reception of an 

 air-cell, but likewise for a fold of the windpipe, which 

 fold expands with age, and lies horizontally in the sub- 



