166 ANIMAL BIOLOGY. [Parti. 



and the vertebral column is terminated by a pygostyle or plough- 

 share bone, composed of several completely ankylosed vertebrae. 



Note the extreme coalescence of parts in the column. The 

 sacrum forms a compact mass, which in the adult is firmly anky- 

 losed to the iliac bones (see Fig. 56, A) by means of the trans- 

 verse processes, coalesced ribs, and anteriorly by the expanded 

 crest of the coalesced neural spines. 



The form of the centrum in birds is noteworthy. The 

 anterior face is convex in vertical section, and concave in hori- 

 zontal section ; while the posterior face is concave in vertical 

 section and convex in horizontal section. In other words, both 

 faces are saddle-shaped, but the anterior saddle is horizontal and 

 the posterior saddle vertical. 



(3.) In the Frog. The vertebral column consists of nine vertebrae 

 and a posterior elongated urostyle. There is no distinction into 

 cervical, dorsal, and lumbar ; but the ninth, the expanded trans- 

 verse processes of which articulate with the ilia, may be regarded 

 as the sacral vertebra. The urostyle articulates with the last 

 vertebra by two concave facets. Along its dorsal surface runs 

 a prominent ridge, in which are two coccygeal foramina for 

 the tenth spinal nerves. The faces of the centra of the second 

 to the seventh vertebras are concave anteriorly and convex pos- 

 teriorly. Both faces of the eighth are concave. The ninth is 

 convex anteriorly .and has two convex tubercles for articulation 

 with the urostyle posteriorly. The atlas has facets for the con- 

 dyles of the skull. The centra contain vestiges of the notochord. 



General Considerations. In studying the successive vertebrae in 

 one organism, or comparing the vertebrae in different organisms, 

 we are struck by the general similarity of plan in the midst of 

 differences of detail. We see specialisation, and yet adherence 

 to type. After considering all the forms presented to us by the 

 vertebrae, we may construct an ideal vertebra, which we may 

 call a typical vertebra. And then we may trace the modifica- 

 tions which have led to greater or less divergence from the 

 typical form. These modifications fall under three heads 

 (Huxley's law) : (1) Coalescence, where parts typically distinct run 

 together and become fused ; (2) Suppression, where parts typically 



