252 'ANATOMY. 



The muscles which run to the plates of the throat may properly 

 be classed with the flexors of the head, for, as the true flexors 

 are attached to these plates, a contraction of these plates likewise 

 draws the head downwards and backwards. There are three on 

 each side : 



One, the flexor of the throat-plate, originates from the inner 

 process of the prosternum, and aflixes itself in the centre of the plate 

 of the throat. 



The second, or straight extensor, affixes itself internally, contiguously 

 to the other, and passes diagonally from the prophragma through the 

 cavity of the prothorax. 



The third, or oblique extensor, comes from the exterior margin of 

 the pronotum, and aflixes itself to the plate of the throat, between the 

 former and the flexors of the head. The two last retain the plates 

 of the throat in their place, which naturally, from the situation of the 

 flexors of the head, is exposed to greater force ; the first assists 

 the head inwards, and also to draw the plate of the throat down, 

 acting in opposition to the two extensors. 



171. 



MUSCLES OF THE MANDIBLES. 



Of the muscles of the joints of the head we will first examine those 

 of the mandibles ; we find two, namely, a flexor and an extensor. 



The flexor of the mandible originates from the entire posterior and 

 upper side of the skull ; it becomes pyramidal and affixes itself, after 

 passing the lateral portion of the brain, by means of a strong and fre- 

 quently divided tendon to the inner margin of the mandible. In many 

 insects, for example, the grasshopper, the entire muscle consists of two 

 contiguous bellies. 



The extensor of the mandible originates beneath the former from 

 the posterior and lower portion of the skull ; it is smaller and weaker, 

 it has a long thin tendon, and affixes itself to the exterior margin of 

 the mandible between the two above-described joint balls. 



The maxillae, which are of a much more complicated structure, have 

 several motive muscles, which may be divided into four groups, 

 according to the part of the maxillae to which they pass. 



There are three muscles which move the entire maxillae. 



The first, the flexor of the maxillae, is the largest ; it originates 

 from the inner side of the throat, closely in front of the occipital aperture. 



