THE INIEEKAL SKELETON. 1 75 



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 Vertebra. — These are all the vertebrae 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- 

 brae 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 vertebrae, which are niostly about eight in number. They ■ 

 have spinous and transverse processes and, at least the hinder 

 ones, develop a hypapophysis which may bifurcate. In the Ehea 

 all the caudal vertebrae 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 Bibs. — Having already noted what are called cervical and 

 sacral ribs, it remains but to consider those which are related to 

 the dorsal vertebrae. 



Eibs 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.—Oi^hese 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. 



