SEC. IV ANATOMY OF BIRDS 219 



sacrarium, .that is, from the first dorsolumbar to the last 

 urosacral. The whole roof -like affair looks something like a 

 keelless stetnum inverted. The pelvic arch of each side consists of 

 three bones, ilmm, ischium, and pubis, which have independent 

 ossific centres, but become firmly consolidated together to form the 

 haunch-bbne or os innominatum. Each of these bones unites with 

 the other two, somewhere near the middle of the whole affair, at a 

 ring-like. structure called the acetabulum (Lat. a vinegar-cruet, Fig. 

 56, a; Fig. 60, arrow ac), or cotyloid, or coxal cavity, which all 

 three cpnsequently contribute to the formation of, and which 

 is the socket for the head of the thigh-bone. When free ribs 

 issue from under cover of the pelvis, they are commonly ankylosed 

 with the ilia ; and all the abortive pleurapophyses of the lumbar 

 and urosacral vertebrae have likewise iliac ankylosis, as explained 

 in treating of the sacrum. As a whole, the pelvis varies like 

 the sternum in relative length, breadth, and degree of convexity ; 

 and especially in the configuration of its posterior border ; but few 

 zoological characters are de- 

 rived from this structure. 



Viewed from below, the 

 pelvis is seen to be much 

 hollowed or excavated for 

 the lodgment of the kidneys, 



J ... , Pig. 61.— Pel™ of young grouse, showing three 



ana cross-cut into compart- distinct tones. 11, is, P, mum, ischium, pubis. In 



ments by the sacral rafters ; w!°sVu"idt" u.l.iT^ ™'*'^''' '''°'™''''' ^°'''^' 

 the series of sacral bodies 



forming a ridge-pole along the middle line. Above, the series of 

 sacral spinous processes represent the ridge-pole; anteriorly, the 

 somewhat spoon-shaped iliac bones are applied, concavity outward, 

 to the dorsolumbars ; posteriorly, in the middle line, is a more or 

 less flattened horizontal expansion, and laterally are the more 

 expanded sides of the ischiac roof, finished along the eaves and 

 behind by the slender pubic bone, which commonly projects back- 

 ward, and inclines toward its fellow of the opposite side. The most 

 prominent formation of the side wall of the pelvis is the thick-lipped 

 smooth articular ring, the acetabulum, converted in the natural state 

 into a cup by a membrane. The postero-superior segment of the 

 rim is prominent, to form the antitrochanter (G-r. avri, anti, against; 

 rpQ-)^avTfip, t'Tochanter of the femur) against which the shoulder or 

 trochanter of the femur abuts when the head is in the ring. 



It is normal to recent Carinate birds to have the ischium fused 

 with the ilium, however distinct the pubis may remain; but to 

 Cretaceous birds (even Ichthyornis), and the existing Ratitce, to have 

 both ischium and pubis distinct in most of their extent. 



The Ilium (Lat. ilium, haunch-bone ; pi. ilia ; adj. iliac ; Figs. 



