III.] GENERAL CHARACTERS. 13 



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 with 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 vertebra. These have been called by Professor 

 Owen zygapopliyses ; that placed on the front edge of the 

 arch being the anlerior zygapopkysis, that on the hinder edge 

 i\\it posterior zys^apophysis. As a general rule the latter have 

 their faces directed downwards, overlying the upward 

 directed anterior zygapopliyses 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 

 zygapopliyses have their faces directed outwards, in which 

 case the corresj^onding anterior zygapopliyses look inwards 

 (Fig. 3, az). 



These articular surfaces on the arch constitute a second 

 mode by which the vertebrae 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 

 long-tailed animals. 



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

 projecting parts c^W^d 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 

 vertebice in a large series of animals, some further light will 



