[CHAPTER XXVIII]

BRANCH CHORDATA (Continued). CLASS MAMMALIA: THE MAMMALS

THE MOUSE (Mus musculus)

Technical Note.—It is best to catch specimens alive in a good trap. A live trap well baited and placed in some old granary should furnish plenty for class use. White mice can often be obtained at "bird-stores." When mice are not procurable, use rats. A rat is perhaps preferable on account of its size, but all essential structures can readily be made out in the mouse. Specimens should be killed by chloroform as described for the toad, p. 5.

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.

Fig. 147.—Dissection of the Mouse, Mus musculus.

Near the hindmost pair of ribs note a sheet of muscles, the diaphragm, which extends across the body-cavity, dividing it into an anterior portion, the thoracic cavity, and a posterior, the abdominal cavity. What are the most conspicuous organs in the thoracic cavity? Leading anteriorly to the mouth-cavity is a long tube, the trachea, composed of a series of cartilaginous parts of rings placed end to end. Note at its anterior end the glottis and epiglottis. Insert a blowpipe into the glottis and inflate the lungs, which will fill all the otherwise unfilled space in the thoracic cavity. The abdominal cavity contains the viscera suspended in a fold of the lining membrane, as in the other vertebrates studied. Note lying against the diaphragm a large, red, glandular structure, the liver. Separate the two large lobes of the liver and expose the opalescent gall-bladder. By passing a canula into this and ligaturing, the cystic duct may be injected. Beneath the liver is a large loop-shaped expansion of the alimentary canal, the stomach. Arising from the right end of the stomach is the narrow duodenum, which gradually merges into the very much convoluted small intestine, or ileum, which is followed by the large intestine, or colon, the last part of which is a straight tube, the rectum. The small intestine occupies most of the space in the peritoneal cavity. Within the loop of the pylorus will be found an irregular pinkish mass of tissue, the pancreas. Beneath the stomach on the left side of the body lies a very dark glandular mass not much unlike the liver but altogether detached from it. This structure is the spleen, a ductless gland.