234 



ANIMAL MECHANICS. 



a second plane be drawn very near the tangent plane, and 

 at the other side of it, this plane will intersect the surface as 

 shown in Fig. 56, not \ 

 meeting the surface 

 within the angles 

 XOA and YOB, but 

 cutting it along the 

 hyperbola contained 

 in the angles XOB 

 and YOA. As be- 

 fore, XY&nd ABare x 

 asymptotes to the hy- Fig. 56. 



peibola, and parallel to the right lines along which the tan- 

 gent plane intersects the surface. The curvatures of the sur- 

 face, in the two angular regions marked out in Figs. 55, 56, 

 &c, are in opposite directions, and, therefore, the perpen- 

 dicular pressures acting on the muscular sheets in these two 



Fig- 57. 



regions are also in opposite directions, while along the lines 



