92 



PRINCIPLES OF ANIMAL BIOLOGY 



Vertebrae articulate with each other chiefly by means of the centra. 

 The articular surfaces of the centra may be concave or convex. Com- 

 monly one of the surfaces of a centrum is concave and the other convex, 

 the convex surface of one vertebra fitting into the concavity of the next. 

 But in some vertebrae both surfaces are concave and the space between 

 the centra is filled with a lens-shaped pad of cartilage. Biconcave verte- 

 brae are called amphicoelous (amphi = both and koilos = hollow). In 



the concavoconvex type of vertebra, 

 if the concavity is directed toward 

 the head, the vertebra is said to be 

 procoelous, but opisthocoelous if the 

 concavity is directed posteriorly. 

 These types of vertebrae are illus- 

 trated in Fig. 81. 



Ribs are usually attached to the 

 vertebrae in such a manner that they 

 can be moved. Some of the hinder- 

 most ribs are free at their ventral 

 ends, while others are connected to 

 the sternum or breast bone more or 

 less directly by means of cartilage. 

 The sternum is a bony or cartilaginous 

 structure which lies in the median 

 ventral part of the thorax. The 

 number of pairs of ribs varies in different species, being 12 in man. 



Parts of Appendicular Skeleton. — The appendicular skeleton consists 

 of the shoulder or pectoral girdle, the hip or pelvic girdle, and the fore 

 and hind limbs. The generalized plan of the girdles and limbs of animals 

 higher than the fishes is shown diagrammatically in Fig. 82. In these 

 appendicular skeletons each of the girdles is composed of three pairs 

 of bones which are similarly arranged in the two gii-dles. Each side 

 of the pectoral girdle is composed of a flat bone, the scapula, or shoulder 

 blade, directed dorsally, a coracoid bone connecting the scapula and the 

 sternum (the latter not shown), and a clavicle which in some vertebrates 

 also connects the scapula and the sternum. There may be a cartilage, 

 the precoracoid, affixed to the posterior edge of the clavicle. A cavity, the 

 glenoid fossa, located at the junction of scapula and coracoid, serves as 

 the surface of attachment of the fore limb. Each side of the pelvic 

 girdle consists of an ilium, ischium, and pubis. These three bones in a 

 generalized skeleton are arranged similai-ly to the bones of the pectoral 

 girdle. The cavity at the junction of the three bones is the acetabulum. 

 In it is seated the head of the femur (thigh bone). 



The bones of the arm and leg or fore and hind limbs are arranged 



A ' \ u \ I c 



Fig. 81. — Three types of vertebrae. 

 Only the centra and lateral processes are 

 shown. Upper end is anterior. A, pro- 

 coelous; B, opisthocoelous; C, amphi- 

 coelous. 



