OSTEOLOGY li'7 



of the reptilian hand, LEIGHTON leans to the view that 

 the supposed pollex is really the index. In putting forward 

 this opinion he rests first of all upon the fact that the radial 

 artery is absent, thus indicating a reduction of the radial 

 side of the hand ; the second argument is derived from the 

 fact that in animals with a reduced manus the first digit 

 is the first to go, and then is followed by the last ; thus in 

 Orohi/>p/tK there are four digits, the first having disappeared, 

 while in Protoliippus the fifth has vanished. In addition 

 this view is moreover strengthened by a consideration of the 

 most reduced manus that occurs in birds ; in Apteryx and 

 Casuarius the reduction has similarly occurred on both sides 

 of the large persisting digit, which is thus to be regarded as 

 No. III. 



Sternum. The sternum in its most complicated condition 

 consists of the following regions (see fig. 72) : Anteriorly 

 it ends in a moderately narrow extremity which is known 

 as the manubrium sterni or rostrum. On either side of this 

 is a forwardly directed process, the costal processes or 

 (interior lateral processes. In the middle of the sternum, 

 and forming the great projecting keel, is the lophosteon or 

 carina sterni, or keel. The sternum ends in a median 

 process behind (sometimes, but wrongly, called the xiphoid 

 process), to which are appended two processes on each side, 

 which may be termed middle and external xiphoid processes, 

 or these may be termed, for reasons which will appear later, 

 the posterior lateral process and the accessory process. The 

 nomenclature first used in the preceding brief descriptions 

 is that of HUXLEY ; the second set of terms which will be 

 used throughout in the descriptions which follow are those 

 used by Miss LINDSAY in her paper upon the development 

 of the avian sternum. 1 



The sternum is subject to much modification among 

 birds, of which the principal varieties will be now described. 

 The birds which show perhaps the greatest difference from 

 the gallinaceous type, selected for the above description, are 

 the ratite birds. In them there is no keel developed, hence 



1 ' On the Avian Sternum,' P. Z. S. 1885, p. 684. 



