ANIMAL MECHANICS. 



319 



Ratio of Work done by the Pectoral Muscle of Birds, round the 

 calculated Axis perpendicular to its Plane, to the maximum 

 Work 



Bird. 



0 



sin <f> 



Albatross, 







0 120 



Wood pigeon, 





9 -35 



0. 166 



Heron, . 





9 .20 



0. 162 



Macaw, . 





2 .00 



0.035 



Pheasant, 





9 .15 



0. 161 





Mean ratio, . . 



. 0-1288 



From these results it appears that the rotation of the wing 

 round the observed axis, perpendicular to the plane of the 

 muscle, gives only ^th part of the maximum work possible ; 

 and that a similar rotation round the calculated axis gives 

 only ith of the maximum work possible. 



This conclusion confirms generally that already arrived at 

 in p. 307, and shows that each muscle is constructed mainly 

 for the performance of a certain kind of work, and that it 

 performs any other kind of work under great disadvantages. 



The condition necessary, in order that a given quadrila- 

 teral muscle should be absolutely incapable of producing any 

 rotation round an axis perpendicular to its plane is readily 

 found. For, the expression (44) for the work done must 

 vanish, which gives us sin 0 = o ; or that the socket must be 

 placed upon the bisector OX (Fig. 84). Hence the centre D of 

 the hyperbola must coincide with 0, and in equation (72) the 

 term involving y must disappear ; hence we have, 



(I + 0 sin 0 



I -I 1 



(77) 



as the condition required, in order that rotation round an 

 axis perpendicular to the muscle shall be impossible. 



