13. When through with the general study of the alimentary canal, you may open the stomach by cutting it lengthwise. Describe the character of the stomach lining as to folds and villi, stating the advantage of each being present and the reason for the direction of the folds. Tell how the food is propelled onward through the alimentary canal. Give the scientific name for this action. _Suggested drawings._ a. The viscera (internal organs) undisturbed. b. The alimentary canal extended. c. The excretory system. The Organs of Circulation and Respiration of the Frog _Materials._ The materials used in this exercise are the same as those used in the preceding exercise. _Observations._ The pupil should identify the following structures:-- _Pericardium_, a membrane that surrounds the heart and, in the case of the frog, separates the body cavity into two portions, the _abdomen_ and the _pericardial_ chamber. _Heart_, lying between the shoulders and in front of the false diaphragm. It is conical in form and composed of three chambers. _Ventricle_, the pink, conical, and muscular portion of the heart, pointing backward and outward. _Auricles_, right and left. These are anterior and dorsal to the ventricle, thin, membranous, and dark-colored. _Arterial trunk_, the single large blood vessel, usually empty of blood, and white. Note its origin and trace it as far as possible, at least until you see it divide to encircle the throat. _Sinus venosus_, a large membranous sac dorsal to the heart and connected with the right auricle. _Lungs_, two small oblong, pink, spongy sacs, lying between and behind the shoulders. The pupil may also identify the following structures if a specimen is available which has the blood vessels injected. _Conus arteriosus_, or "arterial trunk," a large artery passing obliquely forward from the ventricle, and dividing into three branches on each side. _Carotid arteries_, the first branches of the conus, to the head and neck. _Aortas_, the second branches of the conus, to the dorsal region. _Pulmocutaneous arteries_, the third branches of the conus, to the lungs and skin. _Dorsal aorta_, the large artery along the back, formed by the union of the two aortas. _Iliac_, or _femoral arteries_, the two posterior divisions of the dorsal aorta, supplying the legs. _Posterior_ (ascending) _vena cava_, a large vein close to the dorsal aorta, passing forward from the kidneys. _Hepatic veins_, large veins connecting the liver with the posterior vena cava. _Anterior_ (descending) _venæ cavæ_, large veins formed by the junction of the veins from the arm, neck, and head on the right and left sides. _Subclavian veins_, from the arms. _Jugular veins_, from the neck. _Questions._ 1. Of how many chambers does the frog's heart consist? Name them, and describe them as to size, color, and structure.