ANIMAL MECHANICS. 



327 



intersection, will occur when the bones are so placed as to form a 

 quadrilateral inscribable in a circle. 



This remarkable proposition enables us to determine rea- 

 dily the truth or falsehood of the Postulate, p. 238, in several 

 cases. For, if the muscle be really inscribable in a circle, 

 the rectangle under its diagonals must be equal to the sum of 

 the rectangles under its opposite sides, by Ptolemy's Theorem ; 

 and the sides and diagonals may be measured on the fresh 

 subject, by means of a compass, without any serious risk of 

 any great error. 



The Adductor muscles of all animals, as a rule, form skew- 

 sheets of surface, and not plane sheets ; but several of the 

 adductors, in the position of extreme abduction, form plane 

 muscles in the limiting position, and thus enable us to test 

 the truth of our Postulate. 



1 shall number the adductor muscles in the following 

 order for convenience of reference : — 



Adductor Muscles. Human Equivalents. 



1. Adductor primus. 1. The condyloid portion 



of Adductor magnus. 

 1. Adductor secundus. 2. The remaining portion 



of A dductor magnus. 

 a. Ischiadic sheet. (a.) (j3.) 



/3. Pubic sheet. 



3. Adductor tertius. 3. Adductor brevis. 



4. Adductor quartus. 4. Adductor longus. 



5. Adductor quintus. 5. Pectinwus. 



The fourth and fifth of these adductors offer examples of 

 plane quadrilateral muscles, whose origin and insertion lie in 

 rectangular planes. 



1. Lion. — In the Lion the fourth and fifth adductor 

 muscles {longus and pectinwus) have origins and insertions of 

 the same length, and lying beside each other on the bones. 



