THE VERTEBRAL COLUMN. 



19' 



bones, though all constructed on an uniform type, yet do not offer the same 

 configuration throughout the whole rachidean stalk. The differences they 

 present into this respect, have allowed of their being formed into five prin- 

 cipal groups ; whence the division of the vertebral column in five regions, 

 which are, enumerating them from before to behind : 1, Cervical region ; 

 2, Dorsal region ; 3, Lumbar region ; 4, Sacral region ; 5, Coccygeal region. 

 The first comprises seven vertebrse, which serve as a base for the animal's 

 neck : the second has eighteen, against which the ribs are placed ; the third 

 has only six, which correspond to the loins ; in the fourth there are five, 

 constantly solidified into one mass in the adult, to constitute a single bone — 

 the mcrum ; while the fifth possesses a variable number of small degenerate 

 vertebrse, gradually decreasing in size to form the tail. The pieces consti- 

 tuting the first three regions are called true vertebrae ; those of the last two 

 are designated false vertebrae. 



The characters belonging to all these verteorse will be first studied ; then 

 a particular description of the vertebra of each region will be given ; and, 

 finally, an examination will be made of the spine as a whole. 



OHAEACTERS COMMOX TO ALL THE VEETBBE*. 



Each of these small bones is pierced from before to behind by a wide 



Fisr. 10. 



ELEMENTS OP A VERTEBRA: AFTER OWEN. 

 A, Ideal typical vertebra; B, Actual thoracic vertebra of a bird; c. Centrum (or 

 hodiiX giving ofif, d, d, the diapophyses, and p, p, the parapophyses (trcnsverse^ni 

 aHiclilar processes); the neural arch, inclosing the spinal cord, is iormed by 

 „, n, the neurapopllyses (famm<^), and n, ., the neural spme Cspmous process) i 

 the haemal arch, inclosing the great centres of the circulation, is formed by h h 

 the ha^mapoph^ses (costal cartilages), and h, s, the _ha=mal spine (sternum). 

 From both the neurapophyses and htemapophyses maybe given off the zygapophyses, 

 ^ z The lateral arches, which may mclose the vertebral arteries, o, o are 

 completed by the pleurapophyses („/«), i^^. ,- these in Bare bent downwards, so 

 as to form pirt of the hsmil arch, and give off the diverging appendages, a, a. 



opening, the spinal foramen ; whence results, for the entire spine, a long 

 panal traversing its whole length, and which lodges a very important 



