viii INTRODUCTION. 



of a fish or the foot of a turtle metamorphosed into the wing of the bird. 

 But these very diversities of form of the same organ, when carefully 

 examined, present the best proofs of the unity and simplicity of the 

 plan upon which all organic forms are constructed. 



Anterior to the sternum are the clavicles which unite below and 

 form the furcula or merry-thought bone. These are joined to the 

 sternum by ligament or cartilage and the width of these serve to 

 keep apart the shoulders, in opposition to the strong exertions of the 

 muscles of the wing in flight. 



In most birds arboreal birds especially the legs are slender and 

 as light as the wings. These have the long tendons of the flexors and 

 extensors continued to the foot. By the long flexor of the toes passing 

 over the knee and behind the heel, the bending of these joints forces 

 them to grasp mechanically the branches on which they are perched. 



The leg bones consist of a short femur, long tibia with an imper- 

 fect fibula anchylosed to it ; a patella ; an anchylosed tarso-metatarsal 

 bone (the tarsus) and the toes. The pelvis is much extended longi- 

 tudinally, and being anchylosed with the vertebral column, affords a 

 large surface for the attachment of the muscles which support the 

 trunk upon the thighs. The long iliac bones are excavated below 

 and receive the kidneys. The ischia and pubic bones are wide and 

 develope in their cavity the eggs, from which they are also expelled. 



The muscular system of this class is also adapted for their aerial 

 life and to carry them through the atmosphere. According to Pro- 

 fessor Grant, of the Edinburgh College, their irritability or power of 

 contraction is the greatest in the living state, and is the most quickly 

 lost after death, its tenacity after death being generally in the 

 inverse ratio of the degree of activity of that power during life. The 

 muscles are generally more firm and vascular, tougher, stronger, and 

 of a darker colour than in the cold-blooded vertebrates. These 

 properties are most exhibited in the high-flying rapacious birds, and 

 less so in granivorous birds. This muscular force becomes necessary 

 in birds in order that they may fly, either for safety or to pursue 

 their prey through the air, as well as to follow the seasons from 

 latitude to latitude and to perform their migrations over mountain 



