382 BIRDS. 



open on their surface into a number of air-sacs, placed in different 

 parts of the body. 



All birds are oviparous, none bringing forth their young alive, 

 or being even ovo-viviparous. All birds are, lastly, provided 

 with an epidermic covering, so modified as to constitute what are 

 known as feathers. 



The entire skeleton of the Birds is singularly compact, r and at 

 the same time singularly light. The compactness is due to the 

 presence of an unusual amount of phosphate of lime ; and the 

 lightness, to the absence in many of the bones of the ordinary 

 marrow, and its replacement by air. 



As regards the vertebral column, Birds exhibit some very in- 

 teresting peculiarities. The cervical region of the spine is 

 unusually long and flexible, since the fore-limbs are useless as 

 organs of prehension and all acts of grasping must be exer- 

 cised either by the beak or by the hind-feet, or by both acting in 

 conjunction. The number of vertebrae in the neck varies from 

 nine to twenty-four, and their structure is always such as to 

 allow of considerable freedom of motion one upon the other. 

 The dorsal vertebrae vary from six to ten in number, and of 

 these the anterior four or five are generally anchylosed with 

 one another, so as to give a base of resistance to the wings. 

 In the Cursorial Birds, however (such as the Ostrich and 

 Emeu), and in some others (such as the Penguin), in which 

 the power of flight is wanting, the dorsal vertebras are all more 

 or less freely movable one upon another. There are no lumbar 

 vertebrae, but all the vertebrae between the last dorsal and 

 the first caudal (varying from nine to twenty) are anchylosed 

 together to form a bone which is ordinarily known as the 

 " sacrum." To this, in turn, the iliac bones are anchylosed 

 along its whole length, giving perfect immobility to this region 

 of the spine and to the pelvis. 



The coccygeal or caudal vertebrae vary in number from 

 eight to ten, and are movable upon one another. The most 

 noticeable feature about this part of the spinal column is what 

 is known as the " ploughshare-bone." This is the last joint of 

 the tail, and is a long, slender, ploughshare-shaped bone, de- 

 stitute of lateral processes, and without any medullary canal 

 (fig. 330, B). In reality it consists of two or more of the 

 caudal vertebrae, completely anchylosed, and fused into a 

 single mass. It is usually set on to the extremity of the spine 

 at an angle more or less nearly perpendicular to the axis of the 

 body; and it affords a firm basis for the support of the great 

 quill-feathers of the tail (" rectrices "). In the Cursorial Birds, 

 which do not fly, the terminal joint of the tail is not plough- 



