MAMMALS. 355 



filled with marrow. All of the bones except those of the skull, 

 the elements of the sternum, and some of those of the carpus 

 and tarsus, are provided with epiphyses, separate portions 

 which unite later in life with the rest of the bone. 



As a rule five regions cervical, thoracic, lumbar, sacral, and 

 caudal are differentiated in the vertebral column, but in the 

 cetacea, where the sacrum is lacking, the line cannot be drawn 

 between lumbars and caudals. The cervicals, which are almost 

 constantly seven l in number, are free, except in most cetacea and 

 some edentates, where they are greatly flattened and fused. In 

 some rare cases they bear movable cervical ribs, but usually 

 these are firmly fused to dia- and parapophyses, leaving the ver- 

 tebrarterial canal to betray their true nature. Usually the faces 

 of the centra are flat, but opisthoccele vertebrae are common in 

 the necks of ungulates. 



The trunk or dorso-lumbar vertebrae usually number nineteen 

 or twenty, and as a rule, increase in the number of thoracics is 

 correlated with a reduction of the number of lumbar vertebrae. 

 The extremes in the region are found in the armadillo, which has 

 fourteen, and Hyrax, which has thirty dorso-lumbar vertebrae. 

 The number of thoracic vertebrae is usually thirteen, but it is 

 lower in bats and armadillos, and reaches eighteen in the horse, 

 nineteen or twenty in the rhinoceros and elephant, and twenty- 

 three or four in the three-toed sloth. The lumbars vary from 

 two in the monotremes, manatee, and two-toed ant-eaters, to nine 

 in Stenops, the usual number being six or seven. 



The sacral vertebrae are primitively two in number, but 

 others taken from the lumbar and caudal regions may unite by 

 synostosis with the ilium, giving a total number of sacral verte- 

 brae of eight or nine in the sloths and armadillos. The caudals 

 are extremely variable in number and are usually numerous, the 

 number being greatly reduced only in the anthropoid apes and 

 man. 



The ribs (corresponding in number to the thoracic vertebrae) 

 are bicipital, being furnished with tubercular and capitular heads, 

 the former articulating with the diapophysis (the transverse 



1 Manatus australis and Choloepus hoffmanni have six, Bradypus torquatus has 

 eight, and B. tridactylus nine cervicals. 



