194 



ANIMAL MECHANICS. 



muscles, and AB the sternum; OX is the bisector of the 

 angle BOA. 



Using the same notation as in equation (34), we have 



= 8o°, p = 27, 

 0 = 30 0 , q = 10, 



and, finally, 



2Q 



plog, 



cot (25 0 ) 



: O.66. 



2 cot (55 0 ) 



This shows a loss of 34 per cent., arising from the fan-shaped 

 arrangement of the fibres. 



Quadrilateral Muscles — It frequently happens that the 

 fibres composing a triangular muscle do not run on com- 

 pletely to a narrow insertion, but are inserted into a bone or 

 fascia, lying in the same plane with the line of origin. Such 

 a muscle may be called a quadrilateral muscle, and its origin 

 and insertion, like the base of the triangular muscle, may be- 

 come curved lines instead of right lines. The typical qua- 

 drilateral muscle is shown in fig. 39, in which AB, A!B' are 

 the origin and insertion of the muscular fibres. Produce the 

 extreme fibres AA\ BB\ to meet at O, and draw OX'X 



