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inserted on the dorsal surface of the skull. Its lower portion which arises fi'um 

 the pen , forms a thin sheet of muscle in which the post-cephalic cartilage is 

 imbedded. Its upper portion is a thick short sheet of muscle which extends 

 from the ventro-external edges of the nuchal cartilage to the skull. The whole 

 muscle is a cylinder, short and thick above, and long and thin below. 



The nuchal retractor is a thick , strong muscle which arises from the pen 

 in front of the origin of the cephalic retractor and which is inserted upon the 

 dorso-lateral edges and the lower surface of the nuchal cartilage. The cephalic 

 and nuchal retractors move the head and also fasten it to the pen. 



