Examine the _heart_. How many _auricles_ has it? The _ventricles_ in the mouse, as in the bird, are entirely separated, forming two complete compartments, a _right_ and a _left ventricle_. The blood flowing from the veins of the body is collected in the right auricle, thence it passes into the right ventricle, whence it is conveyed to the lungs; returning it flows through the left auricle into the left ventricle, whence it is forced through the arteries of the body. For a study of the circulatory system in mammals (fig. 148), a rat or a rabbit should be injected by the teacher and an advanced text-book, as Parker's "Zootomy" or Marshall and Hurst's "Practical Zoology," used as a guide. A sheep's heart is very good to cut open for a class demonstration. Make a drawing of the organs observed thus far in the dissection. The _kidneys_ in the mouse are situated in the dorsal region next to the backbone. They consist of two bean-shaped smooth glands. From them a pair of ducts, the _ureters_, can be traced down to a median thin-walled muscular sac, the _bladder_. The bladder opens to the exterior of the body by means of a short tube, the _urethra_. Cut open a kidney longitudinally and examine the cut surfaces. The two egg-glands of the female mouse lie in the median portion of the abdominal cavity, somewhat below the kidneys, and from the vicinity of each runs an egg-tube. These tubes meet below the bladder, and open to the exterior of the body through the aperture noted below the anus. In the posterior parts of these tubes lie until birth the developing embryos. TECHNICAL NOTE.--For a study of the nervous system place the specimen ventral side down and cut through the skull with the bone-cutters or heavy scissors, exposing the brain and spinal cord. Note the large _brain_ (fig. 149), composed of small _optic lobes_, large _cerebrum_, _cerebellum_, and _medulla oblongata_, followed by the long _spinal cord_. Note the nerves arising from the brain and spinal cord. [Illustration: FIG. 149.--Diagram of brains of vertebrates; _Olf. L._, olfactory lobes; _Cbr._, cerebrum; _Md. Br._, midbrain (optic lobes); _Cbl._, cerebellum; _Med. Ob._, medulla oblongata; _Sp. Cd._, spinal cord. (From specimens.)] For a careful dissection of the mammalian nervous system a larger mammal, such as a cat or dog or rabbit, should be used. For guide use a text-book such as, for the dog, Howell's "Dissection of the Dog"; for the cat, Reighard and Jennings' "Anatomy of the Cat"; and for the rabbit, Parker's "Zootomy" or Marshall and Hurst's "Practical Zoology." Make a good preparation of the brain and preserve it for future use in some fluid like Fischer's fluid (see page 453). TECHNICAL NOTE.--Prepare a well-cleaned skeleton by boiling a specimen in a soap solution and thoroughly cleansing it (see p. 452).