=Structure= (fig. 147).--Compare the external characters of the mouse with those of the toad and sparrow. The mouse, unlike the other vertebrates so far studied, is thickly covered with _hair_ all over its body except on the tip of the nose and the soles of the feet. Where are the _nostrils_ placed? What are the large leaf-like expansions called _pinnæ_ situated just back of the eyes? Pull open the _mouth_ and note the large _incisor teeth_ on the upper and lower jaws. Cut one corner of the mouth back and observe the large flat-topped _molar teeth_ on both jaws. How does the attachment of the large fleshy _tongue_ differ from the condition in the toad? The toad's tongue is for snapping up insects, whereas in the mouse this organ serves to move food about in the mouth. On the tongue are numerous small _taste-papillæ_. Notice the long hairs, "feelers," on each side of the nose. Note the similarity between the front paws and our own hands; each has four fingers with a small rudimentary thumb on the inner side of the paw. How does the hind foot of the mouse differ from the foot of man? Posteriorly the body is terminated by a long _tail_. At the root of the tail is a small aperture, the _anus_, and just below, or ventral to it, is the opening from the kidneys and reproductive organs. TECHNICAL NOTE.--Place the mouse on its back in a dissecting-pan and cut through the skin from anus to the lower jaw. Extend the legs, pin down each foot and pin out the cut edges of the skin. Now carefully cut forward through the body-wall from the anal region and on through the breast-bones and ribs. Pin each side out.