21 



If I take the triangular muscle running from a bone to a point, and cut 

 off the top of it and place the second bone along the section so cut off, 

 I have a quadrilateral muscle. The quadrilateral muscle is easily dealt 

 with as long as the two bones remain in the same plane. The contrac- 

 tion of the fibres causes these two bones to approach in the same plane. 

 Often, however, A b and a' b' do not always remain in the same plane ; 

 they are not as accommodating as geometers and mathematicians would 

 wish ; they leave their plane and form what we call in geometry skew 

 surfaces (fig. 6), This model intended to show the manner in which 



Fig. 6. 



a plain quadrilateral muscle becomes what we call a skew surface. 

 You are probably acquainted with the term from engineers using it in 

 the construction of what are called skew bridges for i-ailways. A skew 

 surface is a very remarkable thing. It is made up of a number of 

 straight lines, yet every portion of that surface, except along these 

 particular lines, is a curve. Now I have succeeded in discovering that 

 the particular skew surface, of which muscles are capable of assuming 



