86 



tunity of consulting, is a more full or definite exposition of this sub- 

 ject to be found than is contained in the short passage just quoted. 



That the ordinary rules which govern muscular action are not ap- 

 plicable to the genio-hyo-glossi muscles, as protrusors of the tongue, 

 will appear from two considerations : — 



1st. There is no example in the body, unless that furnished by 

 these muscles can be admitted as such, of a muscle carrying its 

 moveable point of attachment beyond its fixed point, by its own 

 contraction. 



2nd. There is absolutely no example in the body, except in the 

 instance of the tongue, of a symmetrical organ, paralysed on one side, 

 and moving, by contraction of its muscles, towards the side of 

 paralysis. 



The point of origin of the genio-hyo=glossus being the superior genial 

 eminence, the course of all its fibres, from origin to insertion, when the 

 tongue is lodged within the mouth, must be more or less directly back- 

 wards, owing to the prominence of the chin (see diagram No. 4). The 

 initiatory stage of the advancement of the tongue, therefore, involves 

 no difficulty of comprehension ; it is effected in accordance with the 

 law of muscular dynamics, by which the extremities of a muscle 

 in contraction tend to approach one another. 



The progress of the tongue beyond the line of the teeth cannot be 

 explained under this law, for it involves the transgression of the fixed 

 point of attachment of the muscles engaged, by their moveable points, 

 and in a ratio proportionate to its advancement ; but without infring- 

 ing this law, the fibres of a muscle inserted at an acute angle into 

 a distant point of a lever may advance that lever in the direction of 

 its axis, or at an angle with it, and in proportion to their length, as has 

 been already shown, and will be understood by reference to diagram 

 No. I . In this law, I conceive, lies the explanation of the protrusion of 

 the tongue uuder the action of the genio-hyo-glossi muscles, to which I 

 now invite the attention of the Academy. 



It has been already shown that these muscles, radiating from a 

 common point of origin on the posterior surface of the body of the 

 inferior maxilla, are inserted into the os hyoides and inferior surface of 

 the tongue along its middle line from base to apex, penetrating its sub- 

 stance even to its dorsum. For the present I leave out of consi- 

 deration the angularity of the planes of the two muscles, as being 

 ' unnecessary to the subject under discussion, namely, the protrusion 

 of the tongue, and in no way qualifying my argument. In the initia- 

 tory stage of protrusion the fibres of the two muscles, having all a 

 direction more or less backwards (see diagram No. 4), co-operate 

 to pull the tongue out of the mouth ; the dorsum is depressed and ren- 

 dered flat ; the tongue becomes rigid and straight ; the os hyoides is 

 raised towards the mouth, and the tip advances beyond the line of the 

 teeth. In the further progress of the tongue the anterior fibres cease 

 to co-operate, maintaining only a state of tonic contraction, and regu- 

 lating the direction of the apex under the guidance of volition. In 

 proportion as the tongue advances a greater number of the fasciculi of 



