=How to tell an Artery from a Vein or a Nerve.=—Raise the suspected vessels to the surface, placing a bone separator underneath to form a bridge, which will cause the blood to recede on every side. If you are in doubt which is the artery, remove the individual sheath from each one. The nerve will appear as a glistening white cord, very solid to the touch and showing bands of nerve fibres which can be separated by the aneurism hook. It will not have any central opening. The vein will appear as a dark blue color and collapsible because of the fact that they have thinner walls than the arteries, lacking the middle circular layer of fibres. Veins have a central opening. As a rule the vein contains blood after death, which gives it the dark blue color but should it not contain blood, it resembles the artery very closely as to color. The vein contains valves which can be seen distinctly, if the blood be pushed the opposite way from which it runs in life. The artery is of a creamish white color and non-collapsible because of the fact that it has heavy walls and a middle circular layer of fibres. The arteries feel firm to the touch while the veins are soft and velvety. Arteries have a central opening and as a rule do not contain blood after death. They likewise do not have valves. The artery is usually more constant than the vein. Should all these not convince you, raise the one you think is the artery with the hook, pass the forceps underneath, spread these and pass the bone separator under for a bridge and with the scalpel incise the artery about one-fourth the way. Attach the arterial tube and if there is blood present, allow it to drain by lowering the arm. When it has ceased to flow, inject very gently and slowly. If you get a half-pint of fluid in the body, you may be assured of its being the artery. Another way to tell the artery from the vein is to roll them lightly between the fingers. If it feels like a thin rubber tube, and does not roll together in a little bundle, the supposition is that the vessel is an artery. This however is not certain, as phlebitis, or some other diseased condition of the veins may result in the thickening of the walls of those vessels, to such an extent as to make it impossible to distinguish in this way between the artery and the vein. The anatomical and the linear guides for the arteries and veins, and their relation to the accompanying nerve, will help also to tell them, one from the other. =How to Cut an Artery for Injection.=—After the artery has been located it should be freed from the surrounding tissues and then raised to the surface. After it has been raised to the surface the bone separator which is to act as a support while cutting the artery, should be placed underneath the artery.