152 STRUCTURE OF THE VERTEBRATES 



mammal. Typically there are seven vertebrae in the mammal, 

 although this number is reduced in some whales, and increased 

 in the edentates and one or two isolated species. However, the 

 giraffe, man, and most others have the same number, no matter 

 how long the neck. All but the first two of these vertebrae bear 

 cervical ribs which are fused by two points to the centrum, and 

 through the canal thus formed runs the vertebral artery. In 

 abnormal cases the cervical ribs (particularly the last two) 

 may be long and movable. 



The first cervical is highly modified to form the atlas which 

 bears the head. The centrum of this vertebra fuses with the sec- 

 ond vertebra in the mammal, and the atlas is left as a bony 

 ring. The second cervical is the axis, the head rotating at the 

 joint between the atlas and axis. The centrum of the first forms 

 the odontoid process of the axis. The posterior five cervicals are 

 very similar in structure. 



Thoracic vertebrae are distinguished by the articular facets 

 for the ribs. The spines of the neural processes are rather long, 

 like those of the cervical ribs, and the muscles and tendons of 

 the occipital region are attached to them. The ribs attached to 

 these vertebrae meet the breast bone, or sternum. 



Lumbar vertebrae of the mammal are heavy and have a stout 

 neural spine. The transverse processes are directed toward the 

 head, and there are no facets for rib attachment. In the lower 

 vertebrates these lumbar vertebrae are hardly distinguishable 

 from those of the thoracic region. 



Sacral vertebrae have the pelvic girdle attached to them. The 

 amphibia have only one vertebra which bears a modified rib 

 for the girdle attachment, while the reptiles have two with 

 sacral ribs. The mammal retains the reptilian number of true 

 sacral vertebrae; but the ribs and the transverse processes of 

 two, three or four other vertebrae become indistinguishably 

 fused to form a heavy sacrum to which the pelvis becomes at- 

 tached. Tlie sacrum of the bird is very large, due to the fusion 

 of seven or more vertebrae. 



Caudal vertebrae are the simplest in structure. In most mam- 

 mals only the first two or three bear any sign of transverse proc- 

 esses, and the more posterior ones consist of only a small cen- 

 trum, the size of this element being reduced from anterior to 



