50 VERTEBRATE SKELETON 



surfaces. Increase in length of the centra with growth of the animal 

 is provided for by separate centres (epiphyses) on either end of the 

 centra, these being but little developed in Monotremes and Sirenia. 

 Other epiphyses are common at the tips of the larger vertebral 

 processes. 



The cervical vertebrae of mammals are remarkably constant in 

 number, the only exceptions to the rule of seven are six in Manatus 

 australis and C holes pus hofmanni, eight in Brady pus torquatus and 

 nine in B. tridactylus . The great variations in the length of the 

 neck (longest in giraffes, shortest in whales) is solely the result 

 of the length of the centra. The longer the neck, the more mobility, 

 and with this there is a reduction of the zygapophyses. The cervi- 

 cals of Cetacea are reduced to thin discs, which, Hke those of some 

 Edentates, are frequently fused. 



The atlas consists of only the neural arch, the 'hypophysial 

 bar,' and an expanded 'transverse process' (cervical rib) perforated 

 by the vertebrarterial canal. It has no neural spine. Its centrum 

 forms the dens of the following vertebra (epistropheus) around which 

 the atlas turns. Many mammals have a foramen in the neural 

 arch for the spinal nerve. The epistropheus has a large neural 

 spine which affords attachment for ligaments connecting the verte- 

 bral column and the cranium. The dens epistrophei (odontoid 

 process) may be cylindrical, conical or spoon-shaped. 



The number of trunk vertebras ranges between i6 in some bats, 

 19 in Artiodactyls, 20-21 in Carnivores, 23 in Perissodactyls and up 

 to 30 in Hyracoids. These are divided into thoracic and lumbar 

 vertebrae, increase in one category in a given region being at the 

 expense of the other. The thoracic vertebrae (varying between 9 in 

 Hyperoodon and Tatusia, 21 in Hyrax and 24 in Cholcepus), are usually 

 13 in number and are characterized by bearing ribs. All have strong 

 neural spines, leaning forwards in front, backwards in the hinder 

 part of the region. The lumbars (varying from 2 to 9) have strong 

 diapophyses, are stout, and, in some groups, may have the additional 

 articulating processes referred to above (p. 49). 



The sacrum (absent from Cetacea and Sirenia) includes from two 

 to eight or ten vertebrae {Glyptodon, Dasypoda) of which but two are 

 true sacrals, the others being synsacrals added from the caudal 

 region. In the sacrum centra and neural arches are often fused, 

 and frequently the neural spines as well. The true sacrals bear 



