SKELETON. 53 



Several of the dorsals are usually fused for strength, but the first presacral is free. 

 A characteristic feature is the synsacrum, foreshadowed in the dinosaurs. As the 

 bird stands on two feet and holds the body obliquely, several of the dorsal and caudal 

 vertebrae (up to 20) fuse with the sacrals into a common mass, a large proportion 

 also uniting with the pelvis. The true sacrals (three in ostriches, two elsewhere) lie 

 just behind the pits occupied by the kidneys and may be recognized by their lower 

 articulation to the pelvis. A few of the caudals behind the synsacrum are free, as 

 all were in Archaopteryx, but the others in recent birds are united into an upturned 

 bone, the pygostyle. 



MAMMALS, except whales where the sacrum is lacking, have all the five verte- 

 bral regions differentiated. With four exceptions the cervicals are seven in number 

 (Manatus anstralis and Cholcepus hofmanni, six; Brady pus torqtiotus, eight; B. 

 tridactylus, nine). The dorsals (thoracics plus lumbars) vary between fourteen in 

 armadillos and thirty in Hyrax, but usually are nineteen or twenty, the number 

 of thoracics usually increasing at the expense of the lumbars. There are primi- 

 tively two sacrals, but others may unite until they amount to nine or ten in some 

 edentates. Usually the centra are amphiplatyan, but in the cervicals of ungulates 

 opisthocoele vertebrae are common. It is to be noted that the 'transverse proc- 

 esses' of the cervical vertebra are, as in birds, composed in part of reduced ribs, as 

 will be shown belo"w. 



RIBS. 



Two different structures are included under the common name of 

 rib, both connected at one end with a vertebra, the other supporting the 

 body walls around the viscera. In following forward the haemal arches 

 in the skeleton of a bony fish (fig. 39,^!, B} it is seen that when the 



FIG. 51. Vertebrae and ribs of (7) anterior and (//) posterior trunk region of Polypterus, 

 after Gegenbaur. p, pleural rib; h, haemapophysial rib. 



body cavity is reached the arch becomes incomplete below, the 

 two haemapophyses separating and coming to lie just beneath the 

 peritoneum in the walls of the ccelom. Above, it is either united 

 directly to the centrum or is jointed to a small process of it. More 

 careful study shows that this fish rib (haemapophysial rib) lies in 

 the intersection of a myoseptum with the median partition of the 

 skeletogenous tissue (p. 38) and is medial to the hypaxial muscles. 

 In the higher vertebrates the rib is formed in the intersection of 



