^ THE FROG 177 



able joints, in which a movement of the bones produces a 

 movement of the body. In other joints the bones are firmly 

 grown together forming the immovable joints. The bones of 

 the skull, for example, are so firmly fused that they 

 appear as a single bone; and the bone of the forearm (Fig. 

 88 r-u) is really made of two bones fused together. Two dis- 

 tinct parts of the skeleton may clearly be seen: (1) the axial 

 skeleton, consisting of the skull and spinal column; (2) the ap- 

 pendicular skeleton, which forms the support for the arms 

 and legs. 



The axial skeleton. The spinal column is composed of 

 nine separate bones called vertebrae; Fig. 88 B. Each ver- 

 tebra consists of a, centrum, c, and a neural arch, na, the 

 arch inclosing the neural foramen (Lat. foramen = opening). 

 From each side of the arch a process of bone extends laterally, 

 called the transverse process (Lat. trans = across -f vertere = 

 to turn) , tr. On the front and back of each vertebra are two 

 smooth surfaces where the successive vertebrae rest upon 

 each other, i. e., articulate (Lat. articulus = joint). They are 

 the articular processes, or zygapophyses. In their natural 

 position the nine vertebras are joined together by their centra, 

 the posterior surface of one touching the anterior surface 

 of the next; Fig. A. The neural foramina are thus placed 

 opposite each other, and all together form a tube which in- 

 closes the spinal cord. The surfaces of the centra fit by a 

 ball-and-socket joint, each of the first seven vertebrae having 

 a ball on the posterior and a socket on the anterior surface, 

 while the eighth is concave on both surfaces, and the ninth 

 is convex on both surfaces. The nine vertebrae are much 

 alike, but can be distinguished from each other. The first 

 has no transverse process, while the centrum of the ninth has 

 two convex posterior surfaces, and very large transverse proc- 

 esses. From the posterior surface of the last vertebra a long 

 slender bone extends backward to the end of the body, the 

 urostyle (Gr. oura = tail + stylos = pillar); Fig. A, ur. The 



