GENERAL STRUCTURE. 



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termed the furcula (fig. 1106). No Bird has more than three 

 digits in the manus ; all of which may be furnished with claws. 

 The three elements of the pelvis are nearly always anchylosed 

 together in the adult (fig. 1102); the ilium being produced in 

 advance of the acetabulum (of which the inner wall is unossified), 

 and the ischium and pubis directed backwards, in a more or less 

 parallel direction, and only very rarely meeting in a ventral sym- 

 physis. The proximal row of the tarsus is always united with the 

 tibia to form a tibio-tarsus ; while the distal row coalesces with the 



Fig. 1102. — Right lateral view of the pelvis and caudal vertebrae of a Fowl (Gallus). II, Ilium 

 Is, Ischium; Pb, Pubis; Am, Acetabulum; dl, Dorso-lumbar vertebrae ; Cd, Caudal do. 



three median metatarsals to constitute a tarso-metatarsus. In all 

 recent Birds only the right aortic arch is present ; the arterial and 

 venous circulations meeting only in the capillaries ; and the blood 

 is hot. 



Some of the above characters are common to certain Reptiles ; 

 and it is only the whole of them collectively which can be regarded 

 as characteristic of Birds as a class. 



Noticing in rather more detail certain features of the osteology, 

 it may be observed that the skeleton is usually remarkable for its 

 combination of compactness and lightness, and also by the per- 

 meation of the greater number of the bones by air-cavities. The 

 skull (fig. 1 103) is formed on the general reptilian type, but is 

 remarkable for the greater relative development of the Drain-case, 

 although this feature is nearly paralleled in the Ornithosauria. The 

 component bones have a great tendency to unite in the adult by 

 the obliteration of their sutures, and their texture is delicate and 

 spongy, and totally unlike the ivory-like structure so generally observ- 

 able among Reptiles. The single occipital condyle, which is mainly 

 formed by the basioccipital, is not placed at the hinder extremity 

 of the cranium, but becomes shifted forwards and downwards, so 

 that the basal axis of the latter forms an angle with the axis of the 



