306 



lique arytenoid muscles appear to be the principal, they are 

 not the *o/e agents in producing the desired adjustment of the 

 cords. The thyro-arytenoid, under certain circumstances, 

 may assist, and also the crico-arytenoid lateralis, as well as 

 the superior fibres of the transverse arytenoid muscle. 



" The general form of lever used in the human body is a 

 lever of the third order, with the muscular insertion so close 

 to the fulcrum, that power is altogether sacrificed to velocity ; 

 but in the instance of the rotation of the arytenoid cartilage 

 upon its horizontal axis, a bent lever of the first order is used, in 

 which there is a great augmentation of power. The extremity 

 of the vertical arm of the lever is at the apex, and of the ho- 

 rizontal arm at the outer angle of the base of the cartilage ; 

 but those two points correspond precisely to the attachments 

 of the oblique arytenoid muscles; and it may be further 

 stated that the incidence of the muscles on the cartilages is 

 most favourable, so that in this particular instance there is 

 scarcely any loss of muscular power. And lastly, it may be 

 observed, that if we do not assign to the oblique arytenoid 

 muscles the special use which we have now delegated to them, 

 they do not appear capable of producing any other motion that 

 could not have been equally well, or indeed more efficiently 

 performed, by the transverse arytenoid muscles." 



The following letter from Sir William R. Hamilton was 

 read, giving some general expressions of theorems relating to 

 surfaces, obtained by his method of quaternions : 



" The equation of a curved surface being put under the 

 form 



f{p) = const. : 



while its tangent plane may be represented by the equation, 



df{p) = 0, 

 or 



S . vdp = 0, 



