458 B I 



one of the most powerful wing, and whether its chief 

 exercise be in long flight, or very rapid flight of 

 shorter continuance, depends a good deal on the 

 number, form, and arrangement of the feathers. 

 Generally speaking, however, birds which do not 

 require the violent rush of the accipitres, ha\e the 

 sternum straighter and narrower, though a little 

 longer in proportion. Sterna which are very broad, 

 with the crest low, always indicate bad fliers. Here, 

 however, the adaptation of the sternum for flight, 

 begins to be combined with that for swimming ; and, 

 as those two habits occur in many proportions with 

 regard to each other, it is not very easy to frame 

 such a general description of the bone as will apply 

 to all or even half the varieties. When birds of this 

 description come to have the two motions of the 

 wings, the flying one in the air, and the swimming 

 one under water, the form of the external part of the 

 wing is adapted to the one or the other, according to 

 the habit, while the form of the sternum itself remains 

 nearly the same. 



Sternal muscles. In order that the action of wings, 

 and the influence which the different modifications of 

 the sternal bones have in that action, may be more 

 clearly understood, it is necessary to say a little more 

 respecting the muscles by which the wings are moved 

 than the mere enumeration which was given in a 

 former section. This may have somewhat the appear- 

 ance of repetition ; but, in treating of complex struc- 

 tures, so much of repetition as shall keep the several 

 parts constantly in the reader's view is not only 

 excusable but necessary ; and when the object is to 

 communicate information, that is not the very best 

 taste which sacrifices perspicuity to mere nicety, and 

 plainness of expression to what is (often very falsely) 

 considered elegance of style. 



The first set of muscles are those which have their 

 insertion in the sternum, the coracoid bone, and the 

 clavicle, and their tendons attached to the humerus ; 

 and these are the muscles which perform the grand ope- 

 ration of working the wing. They are four in number. 



The first, the great pectoral muscle, is situated 

 externally toward the anterior part of the sternum, 

 where it has the freest action. It is inserted on the 

 keel and body of the sternum, on the side of the 

 coracoid, and also on that of the clavicle ; and its 

 tendon, which is particularly strong and firmly united 

 by a large extent of surface, is attached to the 

 extended crest of the humerus on the lower side. 

 When this muscle contracts, it bends down the hume- 

 rus on the shoulder-joint ; but as it is inserted in all 

 the bones by which that joint is supported, the very 

 same action which bends down the humerus on the 

 joint, tends to push the joint upwards, which not only 

 renders the effect of the pulling of the tendon more 

 steady, but actually increases its force. This is the 

 grand* muscle in flying, and it is impossible to imagine 

 an organ better situated or altogether better adapted 

 for its purpose than it is in birds of powerful flight. 



Immediately under this there is situated the middle 

 pectoral muscle, which is the antagonist of the former. 

 This muscle lies in the angle formed by the keel and 

 body of the sternum, and is attached also to the 

 coracoid, and partly to the clavicle on the angle of 

 its inner side. It. passes under the coracoid, and the 

 tendon in which it terminates, passes through a hole 

 which is formed by the uniting of the coracoid, the 

 ciiivtcle, and the scapular, and returning outwards, is 



RD. 



attached, by an extended termination, on the. upper 

 side of the crest of the humerus, rather inward of 

 the centre of motion of the shoulder-joint. This is 

 the principal muscle employed in raising the wing ; 

 and the mode of its action is a little curious. Its 

 insertion is nearly the same as that of its antagonist, 

 the great pectoral muscle ; and when it acts, it tends 

 to raise and keep steady the centre of motion in the 

 shoulder-joint, just as that does ; but as its tendon 

 passes through the pulley, or foramen, above-men- 

 tioned, its power to produce motion is reversed, and 

 though similarly inserted, and as a muscle acting in 

 the very same way, it produces exactly the opposite 

 effect upon the humerus. The tendon which passes 

 through the pulley is, in many birds, in great part 

 ossified, so that, besides having additional firmness 

 from that structure, the upper part of it acts as a 

 more powerful lever in the raising of the wing. 



The action of these two muscles, which are the 

 most important ones in the operation of flying, is one 

 of the most beautiful in the whole range of animal 

 mechanics, wide and varied as it is. The great pec- 

 toral which, by depressing the wing, gives the stroke, 

 and therefore performs the most essential operation 

 in flying, is the most free in its action, as it is pressed 

 by the integuments only. It also pulls the wing 

 downwards with the greatest advantage, as its tendon 

 goes directly to the lower part of the humerus with- 

 out the joint. Its antagonist, which raises the wing, 

 does it by the reversed action of the tendon passing 

 through the pulley ; but it acts more rapidly, and it 

 has the same tendency to support the centre of mo- 

 tion as the other : and though there must be some 

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

 i.s 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 pos- 

 sible on the air when striking downwards, 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 form of joint. It is not a hinge joint, 

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



