MORPIfOLOaY OF IIiailER TYPES 



137 



The forms of the vertebrae in different regions of the vertebral column 

 are very different as shown in Fig. 103. In the thoracic region of an 

 animal having ribs the vertebrae have faces for the articulation of the 

 ribs. In the sacral region the vertebrae in some animals are considerably 

 thickened without great change in form, while in others they are much 

 flattened and more or less fused into a plate-like structure, the sacrum. 

 In the sacral vertebrae the neural canal is reduced in size and in the 

 caudal vertebrae it is entirely absent. 



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 amphiccelous {amphi 

 = both and koilos = hollow) . In the 

 concavo-convex type of vertebra if 

 the concavity is directed toward the 

 head the vertebra is said to be pro- 

 ccbIous, but opisthoccelous if the con- 

 cavity is directed posteriorly. 

 These types of vertebrae are illus- 

 trated in Fig. 106. 



Ribs are usually attached to the 

 vertebrae in such a manner that they 

 can be moved. Some of the hinder- ^ ^na n^u ^ t +i 



Fig. 106. — Three types of vertebrae, 

 most ribs are free at their ventral Only the centra and lateral processes are 



ends while others are connected to «h°^°- Upper end is anterior. ^ pro 



ccelous; B, opisthoccelous; C, amphiccelous. 



the sternum or breast bone more or 



less directly by means of cartilage. The sternum is a bony or cartilagi- 

 nous structure which Hes in the median ventral part of the thorax. The 

 number of pairs of ribs varies in different species. 



Parts of Appendicular Skeleton. — The appendicular skeleton 

 consists of the shoulder or pectoral girdle and the hip or pelvic girdle 

 and the fore and hi7id limbs. The generalized plan of the girdles and 

 limbs of animals higher than the fishes is shown diagrammatically in 

 Fig. 107. In these appendicular skeletons each of the girdles is composed 

 of three pairs of bones which are similarly arranged in the two girdles. 

 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 (not shown in the diagram), and a clavide 

 which in some vertebrates also connects the scapula and the sternum. 

 There may be a cartilage, the precoracoid, affixed to the posterior edge 



