320 



ANIMAL MECHANICS. 



The quantity Q, as I showed before, consists of the dif- 

 ference of two sums, and there is no difficulty in supposing 

 a case in which it should vanish altogether. For the birds 



Quadrilateral and shew muscles, in which the socket of the 

 joint does not lie in the plane containing the origin and insertion 

 of the muscle. 



In page 262, I commenced the discussion of Quadrilateral 

 muscles, in which the plane of the origin and insertion of the 

 muscle includes the socket of the joint, and fully illustrated 

 the action of this important class of muscles by means of the 

 great pectoral of the wings of birds. We have now to 

 examine the class of muscles in which the length of the fibres 

 is such as not to permit the origin and insertion to come into 

 the same plane as that which contains the socket of the joint. 



In this class of muscles, the maximum extension of the 

 fibres possible still leaves the plane passing through the socket 

 and insertion distinct from the plane passing through the socket 

 and origin of the muscle ; and we are to suppose the two bones, 

 each lying in its own plane, placed in the position of maximum 

 extension of fibres, the work to be done being to compel one 

 bone to turn round the other bone, the intersection of the 

 planes containing each being the axis of rotation. 



It is easy to show, as a preliminary condition, independent 

 of any special arrangement of the muscular fibres connecting 

 the two bones, that, in order to produce the maximum moment 

 to turn the planes round their mutual intersection, regarded as 

 an axis of rotation, that it is necessary that the planes passing 



already discussed, its values are — 



Albatross, . . 



Wood Pigeon, . 



Heron, . . . 



Macaw, . . . 



Pheasant, . . . 



0.50 

 3-'9 



1. 12 



