24 ELEMENTARY SKETCH OF THE OSTEOLOGY OF BIRDS. 



corresponds to one of the elements (astragalus)* of the mam- 

 malian tarsus, and that consequently the so-called tibia of a bird 

 is really a tihio-tarsus. The ankle-joint of a bird consequently 

 occurs, not between the tibia and the tarsus, as in mammals, but 

 in the middle of the tarsus itself, f In the succeeding portion of 

 the limb of the bird we find no series of small cuboidal bones cor- 

 responding to the second row of the mammalian tarsus, but on the 

 other hand we have a single long bone (pi. I. t. m. ; pi. IV., fig. 

 5) terminated superiorly by an expanded head (h.) and inferiorly 

 by three semi-distinct pulley-like, or trochlear surfaces (tr.) The 

 study of the development of this bone has shown conclusively 

 that it really consists of a proximal element, which corresponds 

 to the lower half of the mammalian tarsus, and of three 

 parallel long bones, which together form its distal element, and 

 of which the three extremities remain as the trochlese below. 

 These three united bones correspond to the metatarsals of the 

 second, third and fourth digits of the mammalian foot, and the 

 whole bone is consequently called the tarso-metatarsus.X In 

 many birds distinct clefts or vacuities appear in the lower part 

 of the shaft of this bone, which indicate its original composite 

 origin. The fifth digit is invariably unrepresented in birds, 

 and when the first or hallux§ is present (as in the figured skeleton) 

 its metatarsal is generally incomplete above, and is united 

 by ligament to the tarso-metatarsus. In some of the galli- 

 naceous birds, a bony spur is developed from the shaft of the 

 tarso-metatarsus : this spur is ensheathed in horn, and forms a 

 formidable weapon of offence and defence. 



We have now seen the very remarkable differences which 

 exist between the leg of a bird and the leg of a mammal, and 

 before leaving the subject, it may be well to notice that in certain 

 reptiles an intermediate modification of the bones prevails. 

 In living reptiles, as in crocodiles, the tibia and fibula are of 

 equal length, and the latter always articulates with the tarsus. 

 The two rows of the latter remain quite distinct, but the ankle- 

 joint occurs between these two rows. The tibia has no cnemial 

 crest, and the metatarsals are distinct. In certain extinct rep- 

 tiles, however, belonging to the order Dinosauria, the fibula 

 was very slender inferiorly, and the tibia had a large cnemial 

 crest. The astragalus became anchylosed to the tibia to form 

 a tibio-tarsus, but the distal elements of the tarsus and the meta- 

 tarsus, in most cases at all events, remained distinct. We have 



* Astragalus aJrpayaXos the ankle bone. 



f In other words the lower portion of what Ornithologists usually call the Tibia, ia 

 in reality a part of the Tarsus, so that the joint is between two parts of tbia latter. 



X Tarso-metatarsus, compounded of Tapaos and A'^'"'*, after. 

 § Hallux, from allex, the great toe. 



