6 



The Vertebrate Body Skeleton 



Having observed some of the variations of the head 

 skeleton and derived from these certain generalizations 

 relative to the basal radiation of the vertebrate type, the rest 

 of the skeletal system can now be studied. The goals of this 

 examination are the same as those in the previous chapters; 

 specifically these are to learn about the body skeleton and 

 its range of variation in the several kinds of vertebrates, and 

 to evaluate this variation in terms of its contribution to the 

 over-all problem of the evolution (phylogeny) of the 

 vertebrates. 



THE VERTEBRAL COLUMN 

 Mammals 



Although vertebrates get their name from the bony 

 vertebral column, not all of them have such a column. It is 

 perhaps best developed in the mammal. In this group, it can 

 be divided into a number of segments: cervical, thoracic, 

 lumbar, sacral, and caudal. These regions are quite appar- 

 ent in a mounted skeleton, and the terms are defined in the 

 following description. 



The cervical, or neck, vertebrae are distinguished from 

 the other vertebrae by their lack of ribs and by having a 

 vertebral (artery and vein) foramen (transverse canal) in 

 the base of the transverse process. The first two cervicals 

 are highly modified and are called the atlas and axis. There 

 are usually 7 cervicals, but there are exceptions to this 

 number; the manatee has 6, while certain sloths have 6, 8, 

 or 9. 



The thoracic vertebrae bear ribs and usually number 12 

 to 14. The least number is 9 in the whale Hyperoodon, and 

 there are as many as 20 or 21 in Hyrax. 



The lumbar vertebrae lack ribs and the transverse proc- 

 esses are drawn out, down, and forward. The number is 

 variable, depending in part on the number of thoracics. The 

 lumbars complete the presacral series, which usually con- 

 sists of 27 vertebrae. 



The number of sacrals, those fused vertebrae that sup- 

 port the pelvic girdle, is 2 or 3 (5 in man, of which two con- 



tact the ilia). The transverse processes of these vertebrae 

 along with the ventral rib heads (costal centers of ossifica- 

 tion) are expanded and contact the ilium of the pelvic 

 girdle. Vertebrae of the lumbar or caudal series may fuse to 

 the sacral series, thus accounting for variations in the num- 

 ber. The number of caudals is most variable, being least in 

 man with 4, and nearly 50 in the pangolin. Mams, and the 

 insectivore, Mtcrogale. 



Description of vertebrae is difficult because of the func- 

 tional modifications shown in each of the segments of the 

 column and the transitional forms between them. Using the 

 cat or the opossum as representatives, the several kinds can 

 be examined (Figure 6-1). 



In the cat but not the opossum, the atlas has broad flar- 

 ing transverse processes that are perforated basally by the 

 vertebral foramen. This bone is a ring that encloses not only 

 the neural canal, containing the spinal cord, but also the 

 odontoid process of the next posterior vertebra, the axis. 

 Dorsoanteriorly the ring is pierced by bilateral atlantal 

 foramina. There is no neural spine. Posteriorly the ring has 

 large hollowed articular areas for the axis and anteriorly 

 even larger, more deeply hollowed areas for the occipital 

 condyles of the skull. In a young kitten, the atlas is observed 

 to be formed from three ossification centers: two neural 

 arches and a ventromedial intercentrum. \vi intercentrum 

 is missing in some marsupials (wombat and kangaroo): it is 

 a small nodule in the Tasmanian V^o\i (Thalacinus), and is 

 well developed in the opossum. Loss of this intercentrum 

 can be considered a specialization. 



The axis (or epistropheus) is a compound bone with a 

 crest-like neural spine above. At the posterior end of the 

 crest are bilateral and ventrolateral facing postzygapophy- 

 seal articular surfaces. The term "zygapophysis" is derived 

 from the Greek words zygos or zygon which refers to yoking 

 or joining, and apophysis meaning an outgrowth or process. 

 Most of the various processes and articular areas of the 

 vertebrae have names involving the use of apophysis. 



Below the neural canal of the axis is a relatively flattened 

 body, or "centrum," which has a flat, slightly dorsally 

 directed posterior articular surface. Centrum is generally 

 used to identify the body of the vertebra, but it is also used 

 to identify one of the ossification centers involved in verte- 



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