STERNAL MUSCLES. 323 



of the power lost by the reversing of the tendon, that 

 is more than compensated by the steadiness given to 

 the joint, and the increased rapidity of the upward 

 motion. That motion is also greatly facilitated by 

 the structure of the feathered part of the wing, which 

 is so formed that while it takes the greatest hold 

 possible on the air when striking downwards, it takes 

 the least possible in rising upwards. 



Immediately above the mean pectoral muscle, 

 there is a third and much smaller one, which is most 

 conspicuous in those birds which get suddenly on the 

 wing from the ground. Its tendon also passes through 

 the pulley, and is attached to the upper part of the 

 head of the humerus. Its office is to assist the middle 

 pectoral in raising the wing. 



The fourth and last of this group is the small pec- 

 toral muscle, which is inserted on the sternum to the 

 rear of the others, and also partially to the upper 

 and exterior edge of the coracoid. Its tendon is 

 attached to a tubercle on the lower part of the head 

 of the humerus, and its office is two-fold, when the 

 muscles which are more immediately employed in 

 moving the wing during flight, are not in action, this 

 muscle draws the humeral part of the wing toward 

 the body by a peculiar twist of the shoulder-joint ; 

 and when they are in action, it exerts a sort of twist- 

 ing influence, by means of which the posterior edge 

 of the wing is prevented from turning upwards, and 

 a sort of rotatory motion is produced, which has con- 

 siderable influence in the ascent or descent of the 

 bird, and also in the altering of its course while on 

 the wing. 



The articulation of the wing with the coracoid is 

 a very peculiar kind of joint. It is not a hinge joint, 

 neither is it a ball and socket, but something interme- 



Y2 



