Note dorsally of the trachea a long tube passing through the diaphragm and connecting the mouth with the stomach. What is this tube? Note the Eustachian tubes extending from the mouth to the ears. The median part of the roof of the mouth is the palate, hard in front, soft behind. A pair of small bodies at the sides of the soft palate near its hinder end are the tonsils. At the posterior angle of the lower jaw are glandular bodies, the sub-maxillary glands, which lead by a short duct anteriorly to open on the floor of the mouth. On the sides of the neck just below the ears are pink or yellowish bodies, the parotid glands, opening anteriorly in the sides of the mouth-cavity. These two sets of glands are collectively known as the salivary glands, the function of which is to secrete the saliva. Push apart the sub-maxillary glands and note below them overlying the trachea on either side two dark-red lobes connected by a band of tissue. These constitute the thyroid gland, another of the so-called ductless glands. Within the thoracic cavity anterior to the heart note a mass of pinkish tissue, the thymus gland. Observe the large masseter muscles, which cover the jaws. What is their function? On either side of the neck lies a large blood-vessel, the external jugular vein, which collects blood from the head and carries it down to the heart. Note the large pectoral muscles which cover the breast and extend out into the arms, and which are so strong and highly developed in the sparrow. The head is supported by large muscles which run down the back of the neck to the ribs. Others are attached to the ribs, which they raise and lower. These movements, together with the contraction of the diaphragm, cause the expansion and contraction of the thoracic cavity whereby the lungs are regularly filled and emptied. Note that the abdomen is covered by a double layer of muscular tissue, the outer part made up of the external oblique muscles, the inner by the internal oblique muscles.

Fig. 148.—Diagram of
the circulation of the
blood in a mammal; a,
auricles; l, lung; lv,
liver; p, portal vein
bringing blood from the
intestine; v, ventricles;
the arrows show the direction
of the current;
the shaded vessels
carry venous blood, the
others arterial blood.
(From Kingsley.)

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.