TECHNICAL NOTE.--Pass two pieces of thread under the rectum near the pubic bone. Tie these threads tightly a short distance apart and then cut the rectum in two between the threads. Now carefully lift up the alimentary canal with attached organs (liver, etc.), and cut it off near the region of the heart. How is the heart situated with regard to the lungs? The _heart_ consists of a lower chamber with thick muscular walls, the tip, called the _ventricle_, and two upper thin-walled chambers, the _right_ and _left auricles_. Can you make out these three chambers? The purified blood from the lungs flows into the left auricle, while the venous blood from all over the body laden with its carbon dioxide enters the right auricle. From these two chambers the blood enters the ventricle. Here the pure and impure blood are mixed. From the ventricle the blood enters a large muscular tube on the ventral side of the heart. This is the _conus arteriosus_, which gives off three branches on each side; the anterior ones, the _carotid arteries_, supply the head, the next ones, the _systemic arteries_, or _aortæ_, carry blood to the rest of the body, while the posterior vessels, the _pulmonary arteries_, go directly to the lungs and there break up into fine vessels (_capillaries_) where the carbon dioxide is given off and oxygen is taken from the air. From the lungs the blood returns through the _pulmonary vein_ to the left auricle. Meanwhile the blood which has passed through the systemic arteries and body capillaries is collected again into other vessels going back to the heart; these are the _veins_, which empty into a large thin-walled reservoir, the _sinus venosus_, which in turn connects with the right auricle of the heart. Three large veins enter the sinus venosus, namely, two _pre-caval veins_ at the anterior end, and a single _post-caval vein_ at the posterior end. Trace out the larger arteries and veins from the heart to their division into or origin from the smaller vessels. TECHNICAL NOTE.--Carefully remove the heart together with the lungs. The lungs may be inflated by blowing into them through the laryngo-tracheal chamber with a quill and tying them tightly, after which they should be left for several days to dry. When perfectly dry, sections may be cut through them in various places with a sharp knife, and by this means a very good idea of the simple lung structure of the lower backboned animals can be obtained. With a sharp knife cut the heart open, beginning at the tip (ventricle) and cutting up through the conus arteriosus and the two auricles. Note the valves in the heart which separate the different compartments.