THE HEAD AND NECK. 



149 



ditions of life it is necessary that they should remain 

 open in order that the water containing air and nutri- 

 ment may pass freely over the gills and mouth of 

 o 



FIG. 22. The cervical ligament, and bones to which 

 it is attached (from Leisering). 0, occipital crest 

 of skull ; 1 to 7, the seven cervical vertebras ; S, 

 spinous processes of the anterior dorsal vertebrae. 



the animal. They only need to be closed occasionally 

 on the approach of some dangerous enemy. To 

 close the valves and to maintain them in this position 

 requires an effort ; directly this effort is relaxed they 



