52 



too tightly upon the chest, and it also affords surface of attachment to the 

 great pectoral muscle. 



Next to the humerus come the two bones of the fore-arm, the Radius 

 and the Ulna, of which the Ulna is usually the strongest. These two bones 

 are homologous to our fore -arm bones, from the elbow to the wrist. At the 

 elbow of birds there is often found a little bone which is a sort of elbow cap or 

 arm pan. 



Beyond these again we have the wrist or carpus, formed of two short 

 bones ; the metacarpus of two tubular bones which have coalesced at both 

 extremities, and generally two fingers and a thumb ; the thumb is usually 

 nothing but a thin stiliform process, and one finger is always very much larger 

 and longer than the other. 



It will be noticed here by those who have paid any attention to Com- 

 parative Anatomy, that the modifications which I have mentioned above, are 

 such as frequently present themselves in the vertebrate series, and it will also 

 be noticed that those modifications consist of the coalescence of parts and never 

 by their transposition. 



I will pass on to notice very briefly the muscular system by which these 

 bones are set in motion. 



This is extremely similar to that observed in other vertebrate animals. 



The great peculiarity of the muscular system of birds is the enormously 

 developed pectoral muscles ; these muscles arc well known to everybody in 

 the shape of a chicken's breast. They often weigh more than all the other 

 muscles of the body put together. Although resembling one muscle, the mass 

 of flesh upon the breast is really divided into three different muscles. 



The great pectoral is attached to the sternum or breast-bone at one end, 

 and at the other to the humerus or arm-bone, and its function is to depress or 

 pull down the wing. 



It will be easily understood how important a miiscle the pectoral is to birds 

 of prolonged or constant flight, for it is by the action of this muscle chiefly that 

 a bird is enabled to give those powerful and rapid strokes upon the air which are 

 suflScient to sustain it. 



I would also call attention to the fact that the pectoral muscles being so 

 placed that the centre of the bird's gravity is considerably below the line of the 

 outstretched wing, so that in flying a bird has no difficulty in keeping its position, 

 and has no inclination to topple backwards. This feat is sometimes performed by 

 the tumbler pigeon, but it is a work of some difficulty, and the bird almost 

 always requires the assistance of the wind. 



The muscles which raise the wing are the deltoid and the second smaller 

 pectorals. 



The deltoid muscle is attached to the shoulder-blade and to the top of 

 the humerus, and by its contraction raises the wing. 



