260 THE PIGEON 



(b) The Axis is the second cervical vertebra. It has a short odontoid 

 process projecting from the centrum. This process is part of the 

 centrum of the atlas. 



Draw a typical cervical vertebra, e.g., the sixth. 



2. The Thoracic Region. 



This region consists of five vertebrae. The fourth is free, the others are 

 fused. Each bears a pair of well-developed ribs ; and each rib consists of 

 a vertebral portion articulating with the vertebra by a double head (the 

 capitulum and the tubercle), and a sternal portion articulating with the 

 sternum. An uncinate process arises from the posterior edge of the vertebral 

 portion and projects backwards. 



3. The Syn-sacrum. 



Following the fourth thoracic vertebra are thirteen vertebrae all fused 

 together, forming the composite sacral region, the syn-sacrum, and supporting 

 the pelvic girdle. The first of these bears a pair of free ribs and is therefore 

 the fifth thoracic vertebra. The next five have no free ribs and may be 

 regarded as the lumbar vertebrae. These are followed by the two sacral 

 vertebrae. The remaining vertebrae of the fused sacral region are the first 

 five caudals. 



4. The Caudal Region. 



The syn-sacrum is followed by six free caudal vertebrae, and the posterior 

 end of the vertebral column is an upturned and compressed ploughshare bone, 

 the pygostyle, formed by the fusion of the last four caudal vertebrae. 



The following formula shows the composition of the vertebral column of 

 the Pigeon : — 



Syn-sacrum 



A ^ 



Cervical, 14. Thoracic, 4 + 1. Lumbar, 5. Sacral, 2. Caudal, 5 + 6 -1- Pygostyle, 4. 

 Note that the centra of the vertebrae are without epiphyses, i.e., later 

 terminal ossifications. 



The Skull. . 



Make a sketch of the skull viewed from (a) the dorsal surface, (b) the ventral 

 surface, and (c) the side, showing the various bones. 



The skull of the Bird compared with that of the Rabbit shows certain 

 important differences, indicated below by italics. 



Notice that the skull is high and rounded. Many of the bones are ankylosed, 

 the sutures between them being obliterated. 



The cranium (the brain-case) extends forwards, forming the long facial 

 region, which is movable. The large orbits are almost in front of the cranium, 

 and each has a lachrymal bone anteriorly. Between the orbits there is only a 

 thin bony vertical interorbital septum, mainly composed of the mesethmoid 

 and the presphenoid. The cranial cavity does not extend between the orbits into 

 the interorbital septum. The lower border of the interorbital septum is covered 

 by the thin rostrum, a forward prolongation of the basi-sphenoid. The basi- 

 sphenoid is covered by the basi-temporal (which probably represents the 

 posterior portion of the parasphenoid of the Frog ; the anterior portion being 



