THE INTERNAL SKELETON, D5 
become shorter as we proceed backwards, and ultimately grow 
more like those of the hindmost lumbo-sacral vertebrae. There 
may be from five to eleven uro-caudals, as in the Ostrich. 
Caudal Vertebre.—These are all the vertebre posterior to the 
sacrum. They are sometimes called coccygeal, because they 
answer to that terminal part of the spinal column in man 
which is called the coccyx. Almost all existing birds have 
more or fewer—three to ten—of the terminal caudal verte- 
bre anchylosed together into a solid mass called the ** plough- 
share bone,” or pygostyle*. In the Penguin this part is shaped 
somewhat like a man’s wooden leg, and is useful for sustain- 
ing the body in the upright position the Bird assumes when 
on land. Sometimes it expands below into a broad disc 
as in the Woodpecker. It supports the fleshy tail with its 
rectrices and oil-gland. In front of the pygostyle are the free 
caudal vertebrze, which are mostly about eight in number. They 
have spinous and transverse processes and, at least the hinder 
ones, develop a hypapophysis which may bifurcate. In the Rhea 
all the caudal vertebre are distinct, but they are exceptionally 
imperfect in their formation, none having transverse processes, 
and only the first five, or sometimes only three, have complete 
neural arches, which become smaller and smaller from the first 
caudal vertebra, backwards. 
The thoracic part of the axial skeleton consists, as before 
said, of the ribs and breast-bone. 
The Ribs.—Having already noted what are called cervical and 
sacral ribs, it remains but to consider those which are related to 
the dorsal vertebree. 
Ribs belong to two categories :—(1) Those which are dor- 
sally placed and are articulated (save by the rarest exception) 
with the vertebral column, on which account they are called 
vertebral ribs, (2) Those which are ventrally placed, and are 
articulate below with the breast-bone or ‘* sternum,” on which 
account they are called sternal ribs. Each vertebral rib articu- 
lates at its lower end with the upper end of a corresponding 
sternal rib, except the two or three hindmost, which may end 
freely, neither being connected with a sternal rib nor joining 
the sternum. 
Vertebral ribs.\— Of these there are commonly about half a 
dozen on either side, though there may be nine, as in the Diver. 

* See below, p. 195, fig. 157, py. 
