preatlas 



ICi 1C2 



A OPHIACODON 



pro 



1^1 iC2 



B DIMETRODON 



pro 



C CHAMPSOSAURUS D SPHENODON 



nai 



1C2 1C3 



G ALLIGATOR 



E TUPINAMBIS F VARANUS 



Figure 6-5. Anterior vertebrae of reptiles and birds showing their nnodifications for articulation 



with the skull. 



The caudal vertebrae are like the lumbars, but with more 

 slender processes. Neural arches are present along with 

 neural spines on each vertebra right to the tip of the tail. 

 The first two caudals are without intercentra, the third has 

 a small intercentrum in the form of a hemal arch and 

 spine. The fourth has a large hemal arch and spine, while 

 succeeding vertebrae, nearly to the tip of the tail, have sim- 

 ilar, progressively smaller arches and spines. The transverse 

 processes decrease in size posteriorly and disappear at about 

 the middle of the tail. 



In their ossification centers, the vertebrae of the crocodil- 

 ian are much like those of the mammal. The neural arch 

 remains separated from the centrum by a faint line, the 

 neurocentral suture, even in grown animals. The vertebrae 

 of this type differ from those of the mammal in lacking 

 epiphyses and in having the parapophysis on the centrum 

 rather than on the intervertebral disc and the margins of 

 adjacent vertebral bodies. On the basis of the alligator, the 

 costal centers of ossification of the mammalian sacrals are 

 seen to be ventral rib heads. 



Lizard In Tupinambis or Iguana, the vertebral column is 

 much like that of the alligator, except that any trace of the 

 neurocentral suture is lacking. The number of cervicals is 6 

 or 8, depending on how one defines this region. On the basis 

 of short ribs, it is 6; on the basis of full ribs joined to the 

 sternum, it is 8. Using the latter definition, 2 long ribs occur 

 on this first part of the column which do not have direct 



connection with the sternum. In the alligator there is one 

 such large rib, that on the ninth cervical. The first 7 or 8 

 vertebrae (cervicals) have intercentra; those of Tupinambis 

 appear to be functional hypapophyses, closely sutured to the 

 centrum. 



There is no preatlas (Figure 6-5 E,F). The atlas is com- 

 posed of 3 separate pieces in the adult which form a ring widi 

 a wide suture between the tips of the neural arches. The 

 axis is formed of the fused first and second centra, and is 

 nearly fused to the large second intercentrum. The first 

 centrum forms the blunt odontoid process, lying partly 

 within the ring of the atlas. The axis has both pre- and post- 

 zygapophyses and a strong neural spine. 



The third cervical has a transverse process, but no rib, 

 while the fourth may have a short rib. 



From the fifth vertebra on, the ribs are of increasing 

 length. The heads of the ribs are broad with a slight con- 

 striction between dorsal and ventral parts, but they are not 

 "two-headed." There is no vertebral canal through the base 

 of the transverse process. The prezygapophyses of the third 

 cervical and more posterior vertebrae have an accompany- 

 ing overhanging zygosphene. The flange of the postzyga- 

 pophysis of the next anterior vertebrae is grasped between 

 the zygasphene and the prezygapophysis. The cavity around 

 the dorsal articular surface of the inner flange of the post- 

 zygapophysis is called the zygantrum. The articular surfaces 

 should be identified as the pre- and postzygapophyses. 

 There is no evident lumbar series, for all of the vertebrae 



146 



THE VERTEBRATE BODY SKELETON 



