62 COMPARATIVE ANATOMY 



The atlas 1 and axis essentially resemble those of Birds, 

 except that the condylar facet on the former is paired ; in 

 many Marsupials the ventral part of the axis may consist merely 

 of a fibrous band. The differentiation of the vertebral column 

 into regions characterised by difference of form is much more 

 sharply marked than in any other Vertebrates. There are as 

 a rule seven cervical vertebrae ; amongst the Edentata, however, 

 Bradypus possesses eight to nine, and Tamandua bivittata, eight, 

 while in Choloepus (and also in the Manatee) there are only six. 



The transverse processes are simple except in the cervical 

 region and arise from the base of the arch. In the neck, they are 

 united with the vestiges of the cervical ribs, and in nearly all cases 

 enclose a vertebrarterial canal, as in Birds (p. 59): in Monotremes 

 these rib- vestiges remain distinct at any rate for a long time. In 

 the thoracic region the transverse processes are tipped with 

 cartilage on the ventral side of their distal ends for articula- 

 tion with the tubercle of the rib (q.v.). In the lumbar and 

 sacral regions they arise from the centra, and contain fused rib- 

 elements. 



The number of thoraco-lumbar vertebrae varies greatly in 

 different Mammals ; there may be as few as fourteen (Armadillo) 

 or as many as thirty (Hyrax). In Ungulates the number is con- 

 stantly nineteen. In the lumbar vertebra? the transverse processes 

 are especially long, and other processes (anapophyses, metapophyses, 

 hypapophyses) may be characteristically present in this region. 



Thus, as in Amphibians, Reptiles and Birds, the pelvis is con- 

 nected with the sacrum by means of vestigial ribs. As in the two 

 last-mentioned groups, there are not more than two primary sacral 

 vertebra?, but except in Ornithorhynchus and most Marsupials a 

 few caudal vertebrae become later included in the sacrum and are 

 usually more or less closely united with it by synostosis. The 

 various processes of the sacral vertebrae are more or less reduced. 

 In Anthropoids, and still more markedly in Man, the first sacral 

 vertebra is plainly marked off from the last lumbar by the forma- 

 tion of the so-called promontory. A sacrum is wanting in the 

 Cetacea and Sirenia, in correspondence with the absence of hind- 

 limbs : the horizontal tail-fin in these forms is not supported by 

 hard parts. 



The caudal vertebra? vary extremely in their development, and 

 excepting in most long-tailed Mammals such as Kangaroos, 

 Sirenians, Cetaceans, and certain Apes no longer develop lower 

 arches. When present these " chevron bones " are intervertebral 

 in position. 2 The greatest number of caudal vertebra? is found in 



1 A nodule of bone in the atlanto-occipital ligament of the Hedgehog may 

 represent the vestige of a "pro-atlas" (p. 57). 



' The question as to homology of the chevron bones, as well as of certain 

 bony elements present in some Mammals beneath the intervertebral discs in the 

 tail (e.g. Dasypus, Erinaceus) and lumbar region (e.q. Talpa), requires further 

 investigation : it is doubtful to what extent they represent the lower arches or the 

 intercontra of other Vertebrates, or are structures peculiar to Mammals. 



