596 Comparative Morphology of Chordates 



VERTEBRAL COLUMN 



The larger structural characteristics of the mammalian body-wall 

 are clearly reflected in the vertebral column. The differentiation of the 

 five regions of the column is accentuated and they are sharply delimited 

 from one another. In total number of vertebrae and in the relative 

 numbers within the several regions, there is much less variation than 

 among reptiles. Most striking is the fact that the number of cervical 

 vertebrae is almost invariably seven, regardless of the length of the 

 neck. In duckbill, mouse, cat, man, and giraffe the number is seven. 

 In the whalelike mammals, between the skull and the first vertebra 

 whose ribs attach to the sternum (being therefore the first thoracic), 

 there is a bony complex whose structure and mode of development 

 show clearly that it consists of seven vertebrae much flattened and 

 more or less completely fused together so that in some cases the seven 

 occupy scarcely more length in the vertebral column than would a 

 single vertebra of ordinary proportions. The only known departures 

 from seven occur in the Atlantic sea cow (manatee) which has only six, 

 the two-toed sloth (Choloepus hofmanni) with six, and the three-toed 

 sloth (Bradypus) with eight or nine. The first two cervical vertebrae 

 are differentiated as atlas and axis as in reptiles, but with the differ- 

 ence that the anterior face of the atlas is shaped to fit the paired 

 occipital condyles of the mammal. 



The number of thoracic vertebrae is commonly from 12 to 14, 

 and of lumbar vertebrae 5 to 7. But the Asiatic coney (Hyrax) has 

 a total of from 29 to 31 trunk vertebrae. 



In the number of vertebrae involved in the sacral region, there is 

 considerable variation. The bone called "sacrum" usually consists of 

 two or more vertebrae fused together, but some of the fused vertebrae 

 may have no direct connection with the pelvis and therefore may or 

 may not be counted as sacral vertebrae. In a strict sense, a sacral 

 vertebra is one that connects, by means of its pair of short thick ribs, 

 with the ilia. Some statements about number of vertebrae in the sacral 

 region are confusing because it is not clear whether the number refers 

 to the "sacrum" or, in a narrower sense, to the vertebrae which are 

 joined to the pelvis. The egg-laying duckbill has two sacral vertebrae 

 fused together — the typical reptilian number. In kangaroos and many 

 other marsupials two sacral vertebrae constitute the sacrum. But in 

 the Australian bandicoot (Perameles) there is the minimum sacrum, one 

 vertebra only. The wombat (Phascolomys) , another marsupial, is 

 exceptional, having a sacrum of seven vertebrae, but only the anterior 

 four join the ilia. Among placental mammals, with some exceptions, 

 the number of vertebrae in the sacrum ranges from three to five (Fig. 



