12 BRITISH BIRDS 



skull. In the finch tribe there is a slight modification of this, called, 

 from the Greek word for a finch, ' aegithognathous.' In these birds 

 a median bone, called the vomer, from the fact that the bone to which 

 it corresponds in the human skull is shaped somewhat like a plough- 

 share, is truncated in front, instead of tapering, as it does in the 

 schizognathous skull of the common fowl. There is a fourth variety, 

 which marks out the Ostrich tribe and the American Tinamous, in 

 which the two pairs of bones called the pterygoids and palatines do 

 not, as they do in the types of skull that have been hitherto considered, 

 reach the middle line of the skull, but are kept off from it by the 

 vomers, which extend backwards. The term ' dromaeognathous,' or 

 emu-like, is applied to this form of skull. If the back of any bird's 

 skull be examined, it will be noticed that just below the great hole 

 or foramen, through which the medulla passes to join the spinal cord 

 in the canal of the vertebral column, is a rounded, rather kidin-y- 

 shaped boss. This is the occipital condyle, by means of which the 

 skull articulates with the first vertebra. If you look at the same 

 region in a mammal, you will find that there are two of these, one 

 on each side, though also below the foramen magnum. This is one 

 of the many points of structure that distinguish a bird from a 

 mammal and ally it to the reptiles ; but it must be remembered 

 that in some reptiles there is a commencing division of the single 

 condyle into two. 



The Vertebral Column. 



Like all other backboned animals, birds have a chain of small 

 bones running along the back, and enclosing a canal in which runs 

 the spinal marrow. In most vertebrates some of the individual 

 vertebrae in the region of the hind limb, the sacral region, are some- 

 what intimately fused together, forming a more solid structure for 

 the support of the pelvis. In birds the strong coupling of the 

 vertebrae is more marked, and extends to the dorsal region. The 

 mechanical value of this to a flying animal is clear ; it is analogous 

 to the tight coupling of an express train, and prevents the back from 

 bending from side to side under the strain produced by the powerful 

 movements of the muscles in flight. The tail vertebrae show some 

 curious modifications in different birds. In the typical carinate 

 bird, the last few vertebrae are fused into a piece which is called 

 the 'plough-share bone,' or ' pygostyle.' The name of this bone 



