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 vertebrae, 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 sacrum ; while the fifth possesses a variable number of small degenerate 

 vertebras, 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 verteorae will be first studied; then 

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

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



CHARACTERS COMMON TO ALL THE VERTEBRAS. 



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



Fig. 10. 



B us 



ELEMENTS OF A VERTEBRA: AFTER OWEN. 



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

 body), giving off, d, rf, the diapophyses, and p, p, the parapophyses (transverse and 

 articular processes)' the neural arch, inclosing the spinal cord, is formed by 

 n, n, the neurapophyses (lamina;), and n, s, the neural spine (spinous process) $ 

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

 the haemapophyses (costal cartilages), and A, s, the haemal spine (sternum). 

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

 z, z. The lateral arches, which may inclose the vertebral arteries, o, o, are 

 completed by the pleurapophyses (ribs), pi. ; these in B are bent downwards, so 

 as to form part of the haemal arch, and give oft' the diverging appendages, a, a. 



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

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



