BIRDS. 143 



scapular shaft-bone, and the epicoracoid from the coracoidal shaft-bone. The pree-coracoid is always 

 segmented from the head of the coracoid, and the " acromion" (meso-scapula) also gives off a segment 

 in typical Birds neither of these parts are cloven from the common mass in the true Struthionidae. 

 These segments of the Shoulder-girdle are very apt to be fused with the clavicles, for they often 

 borrow their earliest osseous deposit from those subcutaneous splints. In a very large number of 

 Birds the three shoulder-splints are present, forming the fibrous part of the " furcula ;" but 

 that compound bone adopts the cartilaginous segments, as though they originally belonged 

 to it ; and from this arises the great difficulty in comprehending its true nature. In some Birds, 

 for instance, the Rails, some of the Zygodactyli, some of the Syndactyli, and most of Striginse, the 

 clavicles meet without the intervention of the inter-clavicular keystone ; but in some Zygodactyles, 

 Syndactyles, and Owls, the clavicles do not unite with each other. In the Struthious genera 

 Casuariws and Dromaeus the small clavicles do not even approach each other ; in the other genera 

 they are altogether absent, as they are also in certain Parrakeets, for instance, Agapornis, Melo- 

 psittacus, &c., their supposed clavicular rudiments being ossified " meso-scapular segments." The 

 pra3-scapular region is wholly wanting in the Bird, and the continuous meso-scapula (acromion) 

 is very small ; the meso-coracoid is often present, and clasps the attenuated upper end of the 

 " middle pectoral muscle," forming a perfect bony ring round it in some instances, by fusing with 

 the head of the coracoid, just as the meso-scapula fuses with the head of the coracoid, thus 

 embracing the " supra- spinatus muscle " in the Unau and the Megatheroids. 



These modifications of parts of the Shoulder-girdle, as a correlate of the massive development 

 of certain muscles, greatly masks the homology of the parts. 



But the Sternum, the common keystone of the costal arches, is, above all other parts of the 

 skeleton, modified in relation to an inordinate muscular development. The size, both in length and 

 breadth, its outgrowths, its histological consistence, the number of its ossific centres, and, above 

 all, the number of its unfinished clefts all these things conspire to make the study of this part 

 of the Bird's skeleton difficult in the highest degree. My own researches into its nature began 

 twenty-five years ago, and many a struggle have I had with its difficulties, as again and yet again 

 I have returned to it; and even now I have been compelled to seek help 1 in the matter of nomen- 

 clature, in this new essay at its explanation. 



A well-developed Bird's Sternum may be described as made up of three successive regions, as 

 in Man, namely, a " pra3-sternal," a " meso-sternal," and a " xiphi-sternal region : this is its primary, 

 transverse, regional division. Longitudinally, it has Jive parallel regions, the middle region being 

 double, in reality; these are the " lateral/' " intermediate," and " mesial" regions, or tracts. These 

 divisions are marked in various ways, viz. by their relation to surrounding parts, by their ossific 

 centres, and by their arrested clefts. As to the transverse division, there is often a further sub- 

 division to be made out; and this depends upon the formation of "fenestrse," or arrested clefts, 

 tending to cut up the Sternum into succeeding sternal pieces, as in the Mammalia : several mem- 

 branous spaces, of no use to the Teleologist, have this nature, and are a prolepsis of what we often 

 find in a finished condition in the highest Class. 



The longitudinal division of the Sternum is of much greater importance in the Bird than 

 the transverse, yet it is necessary to keep the latter in view for the sake of comparison with the 

 Mammal. If we examine the Gallinaceous Sternum before the middle of incubation (ninth day) 



1 From Professor Huxley. 



