84 



not in the axis of either, but in a line bisecting the angle formed by 

 their prolonged axes (see diagram No. 2, PI. XIII.). 



If, however, the propelling forces be unequal, both levers will de- 

 viate to the side of that which is the weaker ; and if either force be 

 entirely annihilated, then the two levers, though with diminished im- 

 petus, will advance in the prolonged axis of the lever of the unaffected 

 side (see diagram No. 3, PI. XIII.). 



The tongue, as a muscular organ, consists of intrinsic and extrinsic 

 muscles. It would be easy to show, were that necessary to my present 

 purpose, that the principal of the intrinsic muscles — namely, the lin- 

 gual is of Douglas, is connected with the os hyoides. The function of 

 these muscles is to impart to the tongue intrinsic motions, by which its 

 shape and consistence are altered; whilst that of the extrinsic muscles 

 is to communicate to it movements of place and direction, to modify its 

 figure ; and likewise of necessity its density. 



The extrinsic muscles of the tongue are the stylo-glossus, the hyo- 

 glossus, the palato-glossus, and the genio-hyo-glossus ; these muscles are 

 connected, as their names imply, with the styloid process of the tem- 

 poral bone ; the os hyoides ; the soft palate, and the chin, or body 

 of the inferior maxilla, respectively. The stylo-glossus retracts the 

 .tongue, draws it towards the corresponding side, deflects its apex to 

 the same side, and acting in conjunction with the corresponding mus- 

 cle of the opposite side, may expand it transversely, and raise it 

 to the palate. The hyo-glossi retract the protruded tongue whilst 

 contracting it in its transverse diameter, and by depressing its edges 

 they may render its upper surface convex. The palato-glossus may raise 

 the edge of the tongue, and, with the muscle of the opposite side, render 

 its superior surface transversely concave. 



The action of the genio-hyo-glossi is that to which I would invite 

 the special attention of the Academy. These muscles arise from the 

 superior genial eminence of the inferior maxilla, by a common tuft-like 

 tendon, from which the fibres of each muscle expand like the rays of a 

 fan ; the posterior fibres pass backwards and downwards, to be inserted 

 into the body of the os hyoides ; all the other fibres pass through the 

 substance of the tongue, at each side of the middle line, from its in- 

 ferior, towards its superior surface, with various, but successively- 

 diminishing degrees of obliquity from behind forwards ; the anterior 

 fibres, after transversing the substance of the tongue in the direction 

 upwards and backwards for some distance, are curved forward; whilst 

 those immediately in front, which reach the apex of the tongue, are 

 likewise curved slightly downwards in the terminal portion of their 

 course (see diagram No. 4, PI. XIII.). 



The absolute direction of the fibres, from origin to insertion, will 

 be found to vary according to the position of the tongue. "When that 

 organ is entirely confined within the intra- dental portion of the mouth, 

 all the fibres of the genio-hyo-glossus, with the exception of the ex- 

 treme anterior, pass backwards and downwards ; but when the tongue 

 is protruded, or forcibly drawn forwards out of the mouth, the fibres 



