66 



ZOOLOGY. 



evidently constructed on a similar plan. The ribs afford 

 another example ; and in a reduced degree we see the same 

 thing in the general similarity between arms (fore -limbs) 

 and legs (hind-limbs), and the limb-girdles (or bones within 

 the trunk to which the limb-bones are joined). 



The skull, vertebral column, ribs and sternum make up 

 the axial skeleton, as distinguished from the appendicular 

 or limb-skeleton (limb-girdles and limb-bones). 



3. Vertebral Column Thoracic Region. We have seen 



(chap, i., 2) that there are five regions in this column. It 



is convenient to start by 

 examining a vertebra 

 from the thoracic region. 

 In such a vertebra (figs. 

 27, 28) we see a central 

 bony mass, the body or 

 centrum, from which 

 there arises dorsally an 

 arch, the neural arch, 

 completed by a keystone, 

 the neural spine; and 

 arising laterally from the 

 arch are the transverse 

 processes. Looking at the 



vertebra sideways, we see that the arch has a notch in its 



posterior edge : this repre- 

 sents the point where a 



spinal nerve passes out 



from the spinal cord. 



Jointed to the thoracic 



vertebrae on either side 



are the ribs. Each rib 



has a process, the tuber- 



Fig. 27. FIRST THORACIC VERTEBRA AND 

 RIBS. 



Front view. 



-f~ACCTS 



culum, going up to ar- 

 ticulate with the trans- 

 verse process, and one, 

 the capitulum, articulat- 

 ing between the bodies of two contiguous vertebrae. At 

 either end of the body of a vertebra of a young rabbit are 



Fig. 28. THIRD AND FOURTH THORACIC 

 VERTEBRAE. 



Side view. 



