Pruceedivgs. 23 



two bones receive all the attacliments of the great flying- 

 muscles whicli constitute the "cuts off the breast" of the 

 chicken -we have at dumer. The arm of a bird is, as might 

 be expected, modified in such a way as to fit it for a very good 

 organ of flight. First the large humerus or shoulder-bone, 

 then there are two fore-arm bones, the radius and ulna, then 

 follow two wrist-bones, to which are attached three digits, the 

 thumb, fore finger, and middle finger. The thumb is freely 

 movable, and in many birds has a claw at the end. In the 

 Ostrich both the thumb and fore-finger have claws. The 

 bones corresponding to the back of the hand are partly run 

 together and cannot move independently of one another, and 

 it is not at all necessary that they should. 



In the digestive organs of birds probably the most important 

 part is the gizzard. This is a thick, muscular chamber kept well 

 supplied with pebbles, and this arrangement takes the place of 

 the masticating-jaws of other animals. This organ, we have 

 seen, is possessed by the Crocodiles. 



The outer covering of birds — the feathers — is very charac- 

 teristic. They correspond to the scales of reptiles and fish, 

 and the hair on the skin of the higher animals, but are much 

 more complicated in their development, which I will explain. 

 A small pit exists in the skin, and from the bottom of this 

 there projects a long, spongy papilla, which does not remain 

 smooth, but presents upon its outer surface a deep groove in 

 the middle line. From this groove other smaller grooves 

 extend round the papilla, and, gradually becoming shallower, 

 meet on the opposite side. Then other grooves grow between 

 these last grooves. The whole increases in length, and 

 becomes horny and hollow by the withdi-awal of the soft pulp 

 inside ; and when it becomes dry it splits all along its three 

 sets of grooves, producmg what we Icnow as the feather, con- 

 sisting of a hollow, round quill and a square shaft, the shaft 

 bearing a vane or vexillum on each side, such vane being 

 made up of barbs lying close together, and attached to one 

 another by hooks, known as barbules. 



Now, the tail of the ordinary bird terminates in a large, flat 

 bone, which really consists of several joints of the spinal- 



