ENDOSKELETON AND VOLUNTARY MUSCLE 



61 



fore practically absent in the frog. Extending forward from the 

 anterior edge of each neural arch is a pair of spoon-shaped 

 articulating surfaces, the anterior zygapophyses, which face 

 upward and inward. A pair of posterior zygapophyses, projecting 

 backward from the neural arch, faces downward and out, fitting 

 over the anterior zygapophyses of the following vertebra. These 

 articulations between the vertebra at the level of the neural 

 arch together with the ball and socket union between the centra 

 permit a limited amount of movement in the body axis. 



The vertebrae are held together by means of hyaline cartilage 

 between the ends of the centra and by ligaments extending along 

 the sides of the centra and between the neural arches and between 



NS. 



A B 



Fig. 35. — Fourth vertebra of Rana catesbeiana. A, posterior view; B, right 

 side view, az, anterior zygapophysis; c, centrum; na, neural arch; ns, neural 

 spine; pz, posterior zygapophysis; t, transverse process. 



the neural spines. The intervertebral foramina are spaces 

 between adjacent neural arches through which the spinal nerves 

 emerge. 



The first vertebra, or atlas, lacks transverse processes and 

 anterior zygapophyses. In place of the latter there is on either 

 side of the centrum a deep concave surface which articulates with 

 an occipital condyle of the exoccipital bone of the cranium. The 

 ninth vertebra lacks posterior zygapophyses. A pair of rounded 

 knobs, which project from the posterior face of the centrum, 

 articulates with a pair of cavities on the anterior face of the 

 urostyle. The transverse processes of the ninth vertebra are 

 well developed and are attached to the ilia of the pelvic girdle. 

 The urostyle has a small vertebral canal into which the spinal 

 cord extends. The last pair of spinal nerves passes out through 

 small openings near the anterior end. 



In the higher vertebrates the vertebral column is differentiated 

 more completely into regions than is the case in the frog. Thus 

 the cervical region, which in the frog is represented by the single 



