46 COMPARATIVE ANATOMY. 



processes may unite with the corresponding ribs (Fig. 35). In 

 the thoracic region, more or fewer of the vertebrae usually become 

 immovably united together. 



- Pt 



FIG. 35. THIRD CERVICAL VERTEBRA or WOODPECKER (Picus viridis). (Viewed 



anteriorly. ) 



Sa, articular surface of centrum ; Ob, upper arch ; Pa, articular process ; Pt, Pi, the 

 two bars of the transverse process, shown on one side anchylosed with the cervical 

 rib (.B) ; Ft, transverse (vcrtebrarterial) foramen ; Psi, hsemal spine. 



Mammalia. No direct connection exists between the 

 vertebrae of Reptiles and Birds and those of Mammals. The 

 notochord persists longer intervertebrally than vertebrally, but it 

 disappears entirely by the time the adult condition is reached. A 

 jelly-like pulpy mass, the nucleus pulp osus, persists, however, 

 throughout life in the centra of the fibro-cartilaginous menisci 

 which are developed between the vertebrae. Articulations between 

 the centra are never formed, but as in Amphibians, Reptiles, and 

 Birds, well-developed articular processes are present, arising from 

 the neural arches. 1 The cervical region is usually the most 

 moveable, and the centra may be so much hollowed out in this 

 region as to give them an opisthocoelous character (e.g. Ungulata). 

 In some cases, on the other hand, the cervical vertebrae may become 

 firmly fused together (Cetacea). The centra are provided with epi- 

 physes, except in all but those of the caudal region of Monotremes 

 and in Sirenia (?). 



The atlas and axis essentially resemble those of Birds, though 

 the differentiation of the vertebral column into regions, characterised 

 by difference of form, is much more sharply marked than in any 

 other Vertebrates. 



In long-necked Ungulates (Horse, Camel, Ox) the neural spines of the 

 anterior thoracic vertebrae are greatly developed, and corresponding with this, 

 a strong cervical ligament (ligamentum nuchae) is particularly well developed 

 to support the weight of the head. This is also true of antler-bearing animals 

 and of the Gorilla. There are as a rule 7 cervical vertebrae ; Bradypus, how- 

 ever, possesses 8 9, and Tamandua bivittata 8, while in Manatus and Cholojpus 

 there are only 6. 



The transverse processes of the cervical vertebrae usually unite with the 

 rudimentary ribs, as in Birds. 



1 In certain Edentata (e.g. Myrmecophaga) extra articular processes are present 

 besides the ordinary zygapophyses on the posterior thoracic and lumbar vertebrfe. 



