OR OSSEOUS SYSTEM. 85 



observed in the bones of the posterior extremities, but the 

 outer toe is generally quite rudimentary, while the five 

 fingers are more equally developed on the hands. In the 

 aquatic chelonia the bones of the extremities have a more 

 lengthened, straight, and slender form, and especially of the 

 anterior extremities (Fig. 42,) which are much more developed 

 than the posterior. They are also more flat and compressed, 

 and less moveable on each other ; so that they form a near 

 approach to the condition of these bones in the arms of a 

 cetaceous animal, or in the arms and legs of an ichthyo- 

 saurus and a plesiosaurus. The humerus (Fig. 42. d,) the 

 radius (e,) and the ulna (/,) the bones of the carpus (ff } ) and 

 even those of the meta-carpus, and the phalanges of the 

 fingers (h,) partake of this lengthened and flattened form, 

 the best adapted for progressive motion through the water. 

 And we observe the same character, though to a less extent, 

 in the femur (m,) the tibia (n,) and the fibula (o,) and in the 

 bones of the tarsus (p,) the meta-tarsus, and the phalanges 

 of the toes (q,) where all the parts are shorter than the 

 corresponding bones of the anterior extremities. 



XXII. Aves. The bones of birds are more compact, 

 white, dense, and brittle than those of any other class ; 

 they have thinner parietes, their internal cavities are pro- 

 portionally larger, and for the most part they contain air 

 in place of marrow. From the great extent of their res- 

 piration, and the consequent encreased energy of all their 

 functions, ossification proceeds in birds to the greatest ex- 

 tent, not only in the consolidation of the several pieces of 

 the skeleton, but in the anchylosis of the separate elements 

 and separate bones with each other, throughout the skeleton, 

 and in the consolidation, by phosphate of lime, of cartilagi- 

 nous and tendinous parts, not ossified in other classes. 

 In the young state the bones of birds are filled with a thin 

 serous marrow, like those of reptiles, and this is displaced 

 by the admission of air during growth, to a very variable 

 extent, in the different orders of this class, the air being 

 admitted most extensively in the high flying rapacious birds, 

 and least in the heavy swimming palmipeds. There is greater 

 uniformity in the skeleton of this, than of the other ver- 

 tebrated classes. The arms are here adapted solely for flight, 

 the legs for support, and the head and neck are long and 



