96 ORGANS OF SUPPORT, 



the second has four phalanges (2. e,) the third has three 

 (3 /,) the inner toe directed backwards has two phalanges 

 (4. ff 9 ) and the spur seen in the male of many gallinaceous 

 birds is supported by a single osseous phalanx (5.) Where 

 there are only three toes, as in the rhea, and emu, and cas- 

 sowary, the inner toe has three phalanges, and the outer 

 still five ; and in the ostrich, where there are only two toes, 

 the inner toe has four phalanges, and the outer five, as in 

 other birds ; so that the toes are here deficient on the inner, 

 and not on the outer side of the foot where the number of 

 the phalanges remains uniformly the same. In wood-peckers, 

 parrots, cockatoos, and other zygodactylous birds, the outer 

 and the inner toes are both directed backwards, the better 

 to assist in climbing, and consequently one of these has 

 five phalanges, and the other only two. 



XXIII. Mammalia. The bones of mammalia are inter- 

 mediate in density and compactness of texture, and in the 

 extent of their anchylosis between those of birds and those 

 of reptiles. They have generally thick and solid parietes 

 traversed by numerous sutures, which have disappeared 

 in birds, and in the interior of the long bones are large 

 cavities filled with marrow, which in birds are filled with 

 air, and in reptiles with a cancellated structure. The most 

 imperfect forms of the skeleton are presented by the ceta- 

 ceous mammalia, where the vertebral column, as in fishes, 

 is the chief organ of progressive motion, and almost alone 

 developed. They have no sacrum, nor pelvic extremities, 

 and their cervical vertebrae are more or less anchylosed to- 

 gether. Their long bones are almost in the condition of 

 those of reptiles, filled with a loose, internal, cancellated 

 structure, containing a thin, serous, or oily marrow, and all 

 their bones have a coarse, fibrous structure compared with 

 those of land mammalia. The head is still extended in a 

 straight line with the vertebral column, the arms are con- 

 structed for swimming, and the tail is expanded horizon- 

 tally, for the vertical movements of the body, required by 

 their aerial respiration, as seen in the skeleton of the por- 

 poise (Fig. 53.) The bodies of the vertebrae terminate in 

 flat surfaces, united to each other by an elastic, fibro-cartila- 

 ginous, interposed substance, which admits of the necessary 

 movements by means of its compressibility. The terminal 



