THE MUSCULAR SYSTEM. 251 



A. MUSCLES OP THE HEAD. 



170. 



The muscles of the head may be divided into those appropriated to 

 the motion of the whole head and the muscles of the oral organs and 

 antennte. The head has the freest motion of all the moveable parts of 

 the body ; it has thence the most numerous muscles of motion, namely, 

 such which raise it (extensors), such which sink it (flexors), and such 

 which turn it to the right and left (the rotatory muscles). 



The extensors, or raisers of the head (elevatores capilis), are two- 

 fold ; two bellies originate close together from the central line of the 

 pronotum, they somewhat separate in their course, and attach themselves 

 laterally to the margin of the occipital aperture (thence called external 

 extensors, elevator extend). They are shorter and broader than the 

 two other bellies, which come from the prophragma, proceed con- 

 tiguously over the pharynx and through the prothorax, and passing 

 between the preceding affix themselves to the central part of the 

 superior margin of the occipital aperture. All four raise the head up, 

 one acting alone draws it somewhat on one side. 



The flexors, or depressors (depressores capitis), are two small 

 muscles which lie at the under side of the neck, and originate from 

 the neck-plate, or, where this is wanting, from the inner margin of the 

 prostermun, and affix themselves to the lower margin of the occipital 

 aperture. 



Contiguously to them two other small muscles originate, which turn 

 outwardly and attach themselves to the lower part of the lateral margin 

 of the occipital aperture ; they correspond with the anterior bellies of 

 the extensors, and might consequently be called external flexors 

 (depressores eocterni). 



The rotatory muscles of the head (rotalores capitis), are two broad 

 flat muscles, which, coming from the lateral margin of the prosternum, 

 affix themselves to the corresponding margin of the occipital aperture, 

 and bend the head outwardly if one only be in action, but in conjunc- 

 tion they assist to draw the head into the cavity of the thorax. 



In all insects with a free head, (Diplcra, Lepidoptera, Neuroptera, 

 Dictyotoptera, and Hymenoplera,') all these muscles are very small, 

 flat, and like a band ; the following, on the contrary, which belong to 

 the plates of the throat, are, as well as these plates, entirely wanting. 



