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MANUAL OF ZOOLOGY. 



of bony rings or segments, arranged longitudinally, one behind 

 the other. Anteriorly these segments are much expanded, and 

 likewise much modified, to form the bony case which encloses 

 the brain, and which is termed the cranium or skull. Behind 

 the head the segments enclose a much smaller cavity, which is 

 called the " neural " or spinal canal, as it encloses the spinal 

 cord ; and they are arranged one behind the other, forming 

 the vertebral column. The segments which form the vertebral 

 column are called "vertebrae," and they have the following 

 general structure : Each vertebra (fig. 240, A) consists of a cen- 



Fig. 240. A, Lumbar vertebra of a Whale : c Body or centrum ; n n Neural arches ; 

 s Neural spine ; a a Articular processes ; dd Transverse processes. B, Diagram of 

 a thoracic vertebra : c Centrum ; n n Neural arches enclosing the neural canal ; 5 

 Neural spine ; rr Ribs, assisting in the formation of the haemal arch ; // Costal car- 

 tilages ; b Sternum, with haemal spine. (After Owen.) 



tral piece, which is the fundamental and essential element of 

 the vertebra, and is known as the " body " or " centrum " (c). 

 From the upper or posterior surface of the centrum spring two 

 bony arches (n n\ which are called the " neural arches " or 

 " neurapophyses," because they form with the body a canal 

 the " neural canal " which encloses the spinal cord. From 

 the point where the neural arches meet behind, there is usually 

 developed a longer or shorter spine, which is termed the " spi- 

 nous process," or "neural spine" (s). From the neural arches 

 there are also developed in the typical vertebra two processes 

 (a a), which are known as the " articular " processes, or " zyga- 

 pophyses." The vertebrae are united to one another partly by 

 these, but to a greater extent by the bodies or " centra." From 

 the sides of the vertebral body, at the point of junction with 

 the neural arches, there proceed two lateral processes (dd\ 



