128 HIGH SCHOOL ZOOLOGY. 



like Ratitte. Anotlier peculiarity of the hinder extremity is 

 that only the proximal end of the fibida is piesent, and that 

 the ankle joint, as in many reptiles, is situated between the 

 proximal and the distal rows of tarsal bones, the former of 

 which becomes fused with the tibia, into a tibio-tai sus, while 

 the latter and the metacarpals of the second, third and fourth 

 toes becon)e fused into one tarso-metatarse. Wlien the first 

 toe is present, its metatarsal is generally rudimentary, but it 

 has two joints, while the second toe has three, the third four, 

 and the fourth five. The spur of the cock is simply a bony ex- 

 crescence attached to the metatarsus. 



7. It is to be expected that the greatest peculiarities of the 

 muscular system of the birds should be connected with their 

 mode of locomotion. From what has been said above, it will 

 be gathered that the great muscles of flight take their origin 

 from the sternum, and tllus the centre of gravity of the body is 

 shifted towards the most favourable position for flight. Not 

 only the depressor of the wing, but also its elevator muscle 

 arise from the sternum, the necessary change in the direction of 

 the latter being acquired by its tendon passing through a pulley, 

 at the junction of the three bones of the shoulder-girdle, to its 

 insertion in the upper surface of the humerus. In reptiles and 

 mammals where the full number of fingers is present, and the 

 joints of these are freely movable upon each other and on the 

 wrist bones, muscles are necessary for carrying out these move- 

 ments, but the consolidation in the region of the hand of the 

 bird dispenses with the necessity for these and therefore 

 the chief muscles of the fore limbs are in its proximal end 

 near the body. The same is true of the liinder extremity, 

 tendons only being continued into the distal end to carry out 

 the movements of the toes. Thus the great muscles of the 

 limbs are likewise situated, in a favourable position of the body 

 for flight. Cutaneous muscles, chiefly inserted into tho feathers 



