=27.= Go and ask the butcher for a sheep’s pluck. There will most probably be one hanging up in his shop. Look at it before he takes it down. The hook on which it is hanging has been thrust through the windpipe. You will see that the sheep’s windpipe is, like the rabbit’s, all banded with rings of cartilage, only very much larger and coarser. Below the windpipe come the spongy lungs, and between them lies the heart, which perhaps is covered up with a skin and so not easily seen. Hanging to the heart and lungs is the great mass of the liver. When you have got the pluck home, cut away the liver, cut away the skin (pericardium, it is called) which is covering the heart, if it has not been cut away already, and lay the lungs out on a table with the heart between them. You will then have something very much like what [Illustration: FIG. 5.--_Heart of Sheep, as seen after Removal from the Body, lying upon the Two Lungs. The Pericardium has been cut away, but no other Dissection made._ _R.A._ Auricular appendage of right auricle; _L.A._ auricular appendage of left auricle; _R.V._ right ventricle; _L.V._ left ventricle; _S.V.C._ superior vena cava; _I.V.C._ inferior vena cava; _P.A._ pulmonary artery; _Ao_, aorta; _Áó_, innominate branch from aorta dividing into subclavian and carotid arteries; _L._ lung; _Tr._ trachea. 1, solid cord often present, the remnant of a once open communication between the pulmonary artery and aorta. 2, masses of fat at the bases of the ventricle hiding from view the greater part of the auricles. 3, line of fat marking the division between the two ventricles. 4, mass of fat covering the trachea.] is represented in Fig. 5. If you could look through the front of your own chest, and see your own heart and lungs in place, you would see something not so very very different. If now you handle the heart--and if you want to learn physiology you must handle things--you will have no great difficulty in finding the great yellowish tubes marked _Ao_ and _Áó_ in the figure. Your butcher perhaps may not have cut them across exactly where mine has done, but that will not prevent your recognizing them. You will notice what thick stout walls they have, and how they gape where they are cut. _Ao_ is the =aorta=, and _Áó_ is a great branch of the aorta, going to the head and neck of one side, perhaps the branch along which we imagined just now that you, a poor little red blood-corpuscle, were travelling. If you were to put a wire through _Áó_ you would be able to bring it out through _Ao_, or _vice versâ_. But what is _P.A._ which looks so much like the aorta, though you will find that it has no connection with it? You cannot pass a wire from the aorta into it. It also is an artery, the =pulmonary[2] artery=. We shall have more to say about it directly.