THE EVIDENCE FROM MISSING LINKS. 



'55 



9. 



be called "compound ;" since, unlike the simple two-halved lower jaw 

 of quadrupeds, that of birds and reptiles is composed of numerous 

 pieces united to form the single bone. Then, also, this lower 

 jaw is joined to the skull, not of itself and directly, as in man and 

 quadrupeds, but by a special bone named the quadrate, which, 

 curiously enough, by a wonderful process, of alteration and meta- 

 morphosis, becomes represented in man and quadrupeds by one 

 (the malleus] of the small bones of the ear (see page 135). Such, 

 among others, are a few points of agreement between reptiles and birds. 



But plain grounds of distinction are apparent within the same region 

 of " dry bones." A bird has never more than three fingers (thumb [g\, 

 and two next digits \d, f^f~\) in its "hand " or wing (Fig. 75); and the 

 supporting bones of these fingers, correspond- 

 ing to our "palm," are united together. The 

 reptile's fingers are never so few as three, and 

 their pa 1m -bones, moreover, are not ossified 

 together. The "merrythought" of the bird 

 (Fig. 76, /_/), indissolubly associated with 

 mystic forebodings of hymeneal nature, 

 consists of the two united "collar-bones;" 

 such a disposition of the collar-bones being 

 unknown in the more prosaic reptilians ; and 

 the great "keel" (/) seen on the bird's 

 breast- bone (g) is wanting on that of living 

 reptiles. Next in order, we find that the 

 sacrum, or bone wedged in between the 

 haunch-bones, consists, in birds, of a goodly 

 number of vertebrae or joints of the spine, 

 whereas, in the reptile, one or two vertebras 

 form the sacrum. In all birds, save the 

 ostrich tribe, the two haunch-bones (Fig. 76, 

 /, r) are not united below or in front in the middle line. In 

 reptiles such a union does take place, this union, indeed, being 

 also seen in man and quadrupeds. In birds, the tail terminates in 

 a "ploughshare-bone" (Fig. 76, d), giving support to the oil gland, 

 the secretion of which is used in preening the feathers. In reptiles 

 no such bone exists, and the joints of the tail simply taper towards 

 the extremity of the appendage. The axis of the thigh-bone (/) in 

 the bird, like that of quadrupeds, lies parallel with the median plane 

 or axis of the body ; but in reptiles, the axis of the thigh makes an 

 open angle of varying dimensions with the median plane. 



The ankle of the bird (Fig. 77) is peculiarly formed, inasmuch as 

 theupperhalf of the ankle, or " tarsus" (a), becomes united to the lower 

 end of the shin-bone or leg (/) ; whilst the lower half of the ankle 

 unites with the bones corresponding to those of man's instep, the union 



FIG. 

 SKELETON OF 



IRD'S WING. 



