87 



the muscles become inert as regards protrusion, till the final stage 

 is arrived at, which is accomplished by the posterior fibres only, and 

 therefore with greatly diminished force. Retraction of the tongue 

 is now effected by all the fibres of these muscles, whose point of inser- 

 tion is in front of their point of origin, assisted by the special retrac- 

 tors, namely, the hyb, and stylo-glossi (see diagram No. 4). During 

 the progress of the 'tongue forwards the organ is converted into a solid 

 and rigid lever by the antagonistic action of the stylo-glossi, palato- 

 glossi, and hyo-glossi muscles, the two former of which tend to elevate, 

 and the latter to depress it ; whilst the stylo-glossi, by their course 

 along the margins of the tongue to its apex, and acting in equilibrium, 

 render it straight and rigid in its entire length. In this explanation 

 it is impossible to ignore the wonderful selective power which the will 

 possesses, of directing upon special groups of muscles, upon individual 

 muscles, and even upon particular parts of the same muscle, the 

 stimulus of contraction, and in greater or less degree according to 

 circumstances. 



Owing to the fan-like arrangement of the fibres of the genio-hyo- 

 glossi, the anterior fasciculi of the muscles must successively pass out 

 of action as protrusors, according as their points of insertion" are carried 

 in front of the teeth by the advancing tongue ; hence the progress of 

 the tongue forwards must be effected with progressively diminishing 

 power (see diagram No. 4). I have verified this observation in my 

 own person by the following simple experiment : — 



A light wooden cylinder was introduced into my mouth, within 

 the range of my teeth ; the opposite end of the cylinder rested on 

 a balance ; the balance was now weighted, and I found that by pressing 

 the point of my tongue against the end of the cylinder in my mouth, 

 with all the force I was capable of exercising, I could lift a weight 

 of 4lbs. When the tongue was advanced a quarter of an inch in front 

 of the teeth, I could lift 2 Jibs., and when three-quarters of an inch 

 only 21bs. 



No doubt this result may be in some measure explained in another 

 way. It has been shown by Schwann that muscles contract with 

 maximum power in the acme of extension, and with a force diminishing 

 in a progressive ratio as contraction proceeds ; but manifestly so great 

 a difference in the lifting force of the tongue, as that between 4lbs. and 

 2 Jibs., cannot be accounted for in this way. In other words, a loss of 

 nearly one-half the protrusive force of the tongue could not be occa- 

 sioned by a contraction of a quarter of an inch in the posterior fibres of 

 its protrusor muscles. 



In the exhaustive treatise of Bourgery and Jacob, * I find the follow- 

 ing statement : — " As to the comparison of the tw r o genio-glossi mus- 

 cles, since they are united along the middle plane, it will be difficult to 

 apprehend a very perceptible difference between their isolated and 

 simultaneous contraction." 



* Traite Complet de l'Anatomie de l'Homrae, vol. ii., page 53. 



