The Skeletal System 107 



The sacral vertebrae are fused together into the soHd sacrum, 

 which is articulated with the pelvic girdle. The individual vertebrae 

 forming the sacrum can be identified by the openings between them 

 for the passage of the spinal nerves and by the neural spines. 



The caudal vertebrae are highly varied in number in different 

 mammals. They consist of little more than the centrum. In man, 

 the three to five caudal vertebrae are fused together to form the coccyx. 



The number of vertebrae differ in various mammals. 



Man Cat Cozv Rat 



Cervical 7 7 7 7 



Thoracic 12 13 13 13 



Lumbar 5 7 6 6 



Sacral 5 3 5 4 



Caudal 3-5 18-25 18-20 24 or more 



Total 32-34 48-53 49-51 54 or more 



The Thoracic Basket. — The thoracic vertebrae, ribs, and 

 sternum make up the thoracic basket. This bony enclosure provides 

 protection for such important organs as the heart, lungs, and the large 

 blood vessels. In additions, movements of the ribs are important for 

 respiration in mammals. 



A rib (Fig. 32) is a flattened bone that is curved in such a way 

 as to slant down and outward. At its dorsal end, most mammalian ribs 

 have two points of articulation, the head and the tubercle. These are 

 the pivot points. The head articulates on special smooth areas located 

 between centra of adjacent thoracic vertebrae. The tubercle articulates 

 on the end of a transverse process. The narrow region between the head 

 and tubercle is known as the neck and the main part of the rib is known 

 as the slmjt. In man there are twelve pairs of ribs. The cartilages at 

 the ventral ends of the first seven pairs articulate directly with the 

 sternum or breastbone. These are called the true ribs. Each of the 

 ventral cartilages of the next three pairs of ribs articulates with the 

 cartilage of the rib just anterior to it. The last two pairs of ribs, 

 the floating ribs, have no ventral articulations. These last five pairs 

 of ribs are called false ribs. 



The areas between the shafts of the ribs, the intercostal spaces, 

 are filled with the intercostal muscles which aid in moving the ribs 

 during respiration. 



