=External structure.=--Note in the sparrow the same general arrangement of body parts as in the toad, the body being divided into _head_, _upper limbs_, _trunk_, and _lower limbs_. In the toad, however, all of the limbs are fitted for walking and jumping, whereas in the sparrow the anterior pair of appendages, the _wings_, are modified to be organs of flight, and the posterior limbs are specially adapted for perching. Note that the sparrow is covered with _feathers_, some long, some short, in some places thick and in others thin, but all fitting together to form a complete covering for the body. Note also that the anterior end of the head is prolonged into a hard bony structure, the _bill_, covered with horny substance. This horny substance together with the feathers and horny covering of the feet are modified portions of the skin. Note the long _quill-feathers_ attached to the posterior edge of the wing. By these the bird sustains its flight. Other long quill-feathers are attached to the posterior end of the body, forming the _tail_. By a system of muscles connected with these feathers they act together, serving as a rudder during flight and as a balancing contrivance when perching. Note just above the bill two openings protected by tufts of feathers. What are these openings? How are they connected with the _mouth_? Note the large _eyes_, and at the inner angle of each the delicate _nictitating membrane_ which can be drawn over the ball. Does the bird have external _ears_? Lift the feathers just above the tail (the upper tail-coverts) and note a small median gland, the _oil-gland_, from which the bird derives the oil with which it oils its feathers. Beneath the tail note the opening from the alimentary canal and from the kidneys and reproductive organs. This is called the _cloacal opening_.