Anastomosis, the union of the vessels which carry blood or other fluids; also the junction of nerves. The veins and absorbents anastomose to form large single trunks, as they approach their ultimate destinations. The arteries break up into small branches for the supply of the tissues, and each small vessel, again, communicates with others given off above and below. Round each large joint there is very free anastomosis, so that the safety of the limb beyond may not be entirely dependent on the single arterial trunk passing into it, exposed as it is to all the obstructive influences of the different movements of the joint. After the main artery has been permanently obstructed, the anastomosing vessels enlarge, so as to compensate for the loss; but after a time, only those whose course most resembles the parent trunk continue enlarged, and the others gradually regain their ordinary dimensions.
An idea of the profusion of this anastomosing system may be formed from the fact, that if the innominate artery, or great vessel destined for the supply of the right upper half of the body, be tied, and those on the left side injected with size and vermilion, the injection will flow freely into the arteries of the right arm, through branches as minute as they are numerous.