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 
vertebra (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 Archeopteryx, 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 australis and Cholepus hofmanni, six; Bradypus torquotus, 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 
opisthoceele vertebre are common. It is to be noted that the ‘transverse proc- 
esses’ of the cervical vertebre are, as in birds, composed in part of reduced ribs, as 
will be shown below. 
Riss. 
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 hemal arches 
in the skeleton of a bony fish (fig. 39, A, B) it is seen that when the 
Fic. 51.—Vertebre and ribs of (J) anterior and (7Z) posterior trunk region of Polypterus, 
after Gegenbaur. #, pleural rib; 4, hemapophysial rib. 
‘body cavity is reached the arch becomes incomplete below, the 
two hzemapophyses 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 (hemapophysial 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 
