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PAPERS HEAD BEFORE THE ACADEMY, 

 SESSION OF 1867-68. 



XIX. — Ox the Physiology of Pboteusion of the Tongue, and its 

 Deviation to the Affected Side in Unilateral Paralysis. By 

 Thomas Hayden, M.D., M.R.I.A. 



[Read June 11, 1866.*] 

 In the communication which I have the honour of submitting to the 

 Academy I propose to discuss the physiology of protrusion of the 

 tongue, and to endeavour to explain the apparent anomaly by which, in 

 unilateral paralysis of that organ, as exemplified in hemiplegia, it 

 deviates in protrusion to the paralyzed side, whereas the features, as 

 is well known, move to the opposite or unaffected side, as does likewise 

 the tongue itself in all its movements save that of protrusion. 



In order to render intelligible what follows, it will be necessary to 

 start with a few general propositions in reference to the action of vo- 

 luntary muscles. 



Muscular contraction consists essentially in intrinsic molecular ap- 

 proximation, by which the constituent particles of the muscle, its 

 sarcous elements, are brought into closer mutual proximity, and the 

 extremities of the muscle itself are drawn towards one another. 



The range of contraction of a muscle is directly as the length of its 

 fibres, irrespectively of tendon and all other extrinsic substances, and 

 has been variously estimated at one-half to two-thirds of their length. 



The force of the contraction of a muscle is as the number and dia- 

 meter of its fibres, irrespectively of their length ; and its effect depends 

 mainly upon the angle at which it is inserted into the osseous lever ; 

 the order of lever used ; and the point of attachment. 



In no instance can a muscle in contraction carry its moveable, be- 

 yond its fixed point of attachment. 



A muscle acting upon a lever at an acute angle, and moving it in the 

 direction of its axis, may, however, carry* the proximal extremity of 

 the lever far beyond its own fixed point of attachment, the distance 

 being regulated by the length of the lever, and the length of the fibres 

 of the muscle (see diagram JSTo. 1, PI. XIII.). 



Two levers so acted upon by two coequal forces, and moving at an 

 acute angle, say of 45°, would have a tendency to intersect at their 

 point of mutual contract ; if inflexible, and offering equal resistance, 

 they would both be arrested at this point ; but if flexible, of equal 

 •power of resistance, and propelled by equal forces, they would advance, 



* This paper was held over for the " Transactions," but the author not wishing to 

 leave it any longer unpublished, it is printed here, though not properlv belonging to the 

 Session of 1867-68. 



R. I. A. PROC. VOL. X. N 



