PHYLUM CHORDATA 



433 



The bones of the hind-Iimh are 

 class, but the form of the 

 tarso-metatarsus of Penguins 

 is -worthy of notice. It is 

 short and wide, its three con- 

 stituent metatarsals, though 

 fused, are clearly distinguish- 

 able throughout their whole 

 length, and tbe resemblance 

 to the homologous part in 



very 



uniform throughout the 





Pig. 1072.— Gallus bankiva (common Fowl). 

 Innominate of a six days' embryo. Jl, ilium ; 

 .7s, ischium ; ph. pubis ; jtp. pectineal process. 

 (From Wiedersheim's Comparative Anatomy, 

 after Johnson.) 



Iguanodon is very striking. 



In the embryo (Fig. 1073) a 



vestige of the fifth digit (Mt. 



tsl. 5) has been found in the 



form of a small rod of cartilage 



on the postaxial or fibular 



side. One or two free cen- 



tralia may occur in the meso- 



tarsal joint. 



The skeleton is always more or less pneumatic, but there is no 



definite relation between pneumaticity and power of flight. A very 



usual arrangement is for all the 

 bones to contain air except those 

 of the fore-arm and hand, shank 

 and foot. But ia Apteryx, Pen- 

 guins, and some Song-birds the 

 skull alone is pneumatic, while in 

 the Hornbill every bone in the 

 body contains air. 



Myology. — As might be in- 

 ferred from a study of the skele- 

 ton, the muscles of flight undergo 

 a great reduction, often amount- 

 ing to complete atrophy, in the 

 Ratitse ; and to a less degree in 

 the flightless Carinatse. The pre- 

 sence or absence of an ambiens 

 and of certain other muscles in 

 the leg and in the wing furnish 

 characters of considerable classi- 

 ficatory importance. 



Digestive Organs. — In all 

 existing Neornithes the jaws are 

 covered by a horny beak and 

 there are no teeth. But that 

 teeth were present in the more 



primitive Birds, and have gradually been lost during the evolution 



Fin. 1073.— Apteryx oweni. Left hind- 

 limb of embryo, dorsal aspect. Oist. 

 distale ; Fe. femur ; /■'». fibula ; jib. fibu- 

 lare ; Mt, tst. 1 — 5, metatarsals ; Tib. tibia ; 

 tfb. tibials. (After T. J. Parker.) 



