PRINCIPLE OF LEAST ACTION 91 



from a fixed line and converge to a very short one, 

 practically a point ; (4) quadrilateral is the same as the 

 triangular, but in which the short line is longer, so that 

 the whole makes a four-sided figure ; (5) sphincter and 

 ellipsoidal, which surround a circular or elliptical space". 



The quadrilateral muscle is peculiar in that if the 

 bones be not in the same plane the fibres instead of 

 being parallel now take up the position forming a " skew 

 surface" precisely on the same principle as of a skew 

 bridge. Dr. Haughton observes : " I have succeeded in 

 discovering that the particular skew surface, of which 

 muscles are capable of assuming the shape, is the 

 beautiful surface known to geometers by the name of 

 the hyperboloid of one sheet. 



" This is not a mere fiction of geometers. The adductor 

 magnus muscle in the leg of man, and the great pectoral 

 muscle in the wing of every bird, are living examples of 

 the reality of this curious fact, that Nature constructs 

 not merely plane muscular structures but that she is 

 capable of constructing muscular surfaces belonging to 

 the most beautiful and elegant forms that have been 

 studied and invented by abstract geometers. 



" The prismatic muscle and the penniform muscle 

 possess the remarkable property, which can be demon- 

 strated mathematically, that in their contraction no loss 

 whatever takes place . . . whereas in the triangular, 

 quadrilateral and skew muscles we can demonstrate that 

 in the use of every such muscle there is a necessary loss 

 of force ; but these are only used under great necessity. 



" The most wonderful triangular muscle in the world 

 is the biceps femoris or the flexor of the thigh of a tiger. 

 In the leg of a man it is like a rope of parallel fibres, 

 but a tiger's habit is to leap from the jungle at its prey. 

 The weight of muscle to give the tiger the spring is 



