363. Injuries to the Blood-vessels. It is very important to know the difference between the bleeding from an artery and that from a vein. If an artery bleeds, the blood leaps in spurts, and is of a bright scarlet color. If a vein bleeds, the blood flows in a steady stream, and is of a dark purple color. If the capillaries are injured the blood merely oozes. Bleeding from an artery is a dangerous matter in proportion to the size of the vessel, and life itself may be speedily lost. Hemorrhage from a vein or from the capillaries is rarely troublesome, and is ordinarily easily checked, aided, if need be, by hot water, deep pressure, the application of some form of iron styptic, or even powdered alum. When an artery is bleeding, always remember to make deep pressure between the wound and the heart. In all such cases send at once for the doctor. Illustration: Fig. 157.—Showing how Digital Compression should be applied to the Femoral Artery. Do not be afraid to act at once. A resolute grip in the right place with firm fingers will do well enough, until a twisted handkerchief, stout cord, shoestring, suspender, or an improvised tourniquet[53] is ready to take its place. If the flow of blood does not stop, change the pressure until the right spot is found. Sometimes it will do to seize a handful of dry earth and crowd it down into the bleeding wound, with a firm pressure. Strips of an old handkerchief, underclothing, or cotton wadding may also be used as a compress, provided pressure is not neglected. In the after-treatment it is of great importance that the wound and the dressing should be kept free from bacteria by keeping everything surgically clean. 364. Where and how to Apply Pressure. The principal places in which to apply pressure when arteries are injured and bleeding should always be kept in mind. Experiment 192. _How to tie a square knot_. If the student would render efficient help in accidents and emergencies, to say nothing of service on scores of other occasions, he must learn how to tie a square or “reef” knot. This knot is secure and does not slip as does the “granny” knot. The square knot is the one used by surgeons in ligating vessels and securing bandages. Unless one knew the difference, the insecure “granny” knot might be substituted. A square knot is tied by holding an end of a bandage or cord in each hand, and then passing the end in the _right_ hand over the one in the left and tying; the end now in the _left_ hand is passed over the one in the right and again tied. Illustration: Fig. 158.—Showing how a Square Knot may be tied with a Cord and a Handkerchief. If in the finger, grasp it with the thumb and forefinger, and pinch it firmly on each side; if in the hand, press on the bleeding spot, or press with the thumb just above and in front of the wrist.