20 



thing is curved in forms that mathematicians cannot imitate. We find 

 the muscular fibres so arranged that, if A b and a' b' lie in the same plane, 

 if I produced them all, they would pass through a common point of 



Fig. 5- 



intersection. I ask you to take that for granted. I have proved it, but 

 I will not trouble you with the proof. Therefore the quadrilateral 

 muscle is really nothing but a triangular muscle with the top cut off. 



