1 8 THE VERTEBRAL COLUMN. [CHAP. 



less open angle. At the point of their junction there is 

 usually a single median process projecting dorsally, called 

 the spinous process or neural spine. 



In most cases upon the anterior and posterior edges of the 

 laminae of the arch are flattened, slightly projecting, more or 

 less oval, smooth surfaces or facets, which in the natural state 

 are covered by a thin layer of cartilage, and come into 

 contact and articulate (by synovial joints) with the corre- 

 sponding surfaces of the immediately antecedent and suc- 

 ceeding vertebrae. These have been called by Sir Richard 

 Owen zygapophyses ; that placed on the front edge of the 

 arch being the anterior zygapophysis, that on the hinder 

 edge the posterior zygapophysis. As a general rule the latter 

 have their faces directed downwards, overlying the upward 

 directed anterior zygapophyses of the vertebra next behind. 

 This is a useful rule to remember in ascertaining which is 

 the front and which the posterior surface of a vertebra. 

 Sometimes, especially in the lumbar region, the posterior 

 zygapophyses have their faces directed outwards, in which 

 case the corresponding anterior zygapophyses look inwards 

 (Fig. 3, az). 



These articular surfaces on the arch constitute a second 

 mode by which the vertebras are united, and their size and 

 conformation aid to regulate the amount of motion allowed 

 between the component parts of the column. They are often 

 entirely wanting when flexibility is more needed than 

 strength, as in the greater part of the caudal region of 

 lonor-tailed animals. 



o 



In addition to the body and the arch, there are certain 

 projecting parts called processes, more or less developed in 

 different vertebrae. Many difficulties exist about the signi- 

 fication, homologies, and terminology of these processes. 

 Probably, when more is known of the development of the 



