326 AN INTRODUCTION TO THE STUDY OF FOSSILS 



n.sp. 



attached the hyoid bones, which support the larynx, or voice 

 box. To the backbone ventrally are joined- the ribs, which 



with the breastbone (sternum) 

 protect the heart and lungs. 



Primitively, as in most fish 

 (Fig. 149) and snakes, a pair of 

 ribs is attached to each vertebra 

 from the base of the head into 

 the tail region. But with up- 

 right land life they have become 

 reduced to a small part of the 

 trunk region. Even in the 

 higher mammals, however, there 

 occur in the embryo beginnings 

 of the other ribs coalescing later 

 with the vertebrae. 



The backbone, or vertebral 



Fig. 140. — Posterior view of the fifth 

 cervical vertebra of the cat; to illus- 

 trate nomenclature, a.z., anterior 

 zygapophysis, — a flattened articu- 

 lating surface upon the neural arch 

 ipe. and la. combined), looking down- 

 wards upon the posterior zygapoph- 

 ysis of the vertebra ahead. This 

 a.z. at times, especially in the lumbar 

 region, looks inwards; c, centrum; 

 la., lamina, — transverse portion of 

 the neural arch. Its anterior and 

 posterior faces, called zygapophyses, 

 are flattened for articulation w^ith 

 neighboring vertebrae; n.c, neural 

 canal, lodging the spinal cord; n.sp., 

 neural spine ; p., transverse process, 

 sometimes divided into a superior 

 and an inferior branch ; pe., pedicle, 

 — the vertical portion of the neural 

 arch, notched upon the side for exit 

 of nerves from the spinal cord ; p.z., 

 posterior zygapophysis ; this looks 

 downward (in the lumbar region 

 often outward) ; v.c, vertebrarterial 

 canal, piercing the base of the trans- 

 verse process. In the caudal region, 

 especially where the tail is well de- 

 veloped, paired bones, uniting at 

 times to form an arch, arise upon the 

 ventral side of the centrum ; these 

 are the chevron bones. (Redrawn 

 from Mivart.) 



column, is a 



hollow rod lodging 

 the spinal cord, and divided 

 into some thirty to fifty sec- 

 tions, or vertebrae ; i.e. seven 

 neck (cervical. Fig. 140), thirteen 

 chest (thoracic), with attach- 

 ment for the thirteen pairs of 

 ribs, seven back (lumbar), three 

 hip (sacral) and four (in the 

 manx cat) to twenty-six tail 

 (caudal) vertebrae. 



In most mammals the num- 

 ber of cervical vertebrae is 

 seven ; the long neck of the 

 giraffe has thus been produced 

 merely through the lengthening 

 of the individual vertebrae. With the swan, among the birds, 

 however, there has been an increase in their number to twenty- 

 three and with this a consequent increase in grace of movement. 



