Anastomosis (Gr. ava, through, and mouth), the communication or inosculation of different blood vessels by opening one into the other. In the arteries it is comparatively rare, as these vessels divide and separate from each other, for the purpose of distributing the blood to different organs. Nevertheless, it always exists in certain situations, where the principal trunk is liable to compression, and where this compression would have the effect of shutting off all nourishment from the parts beyond were there no other means of vascular communication. Thus the arterial branches situated above and below the principal joints anastomose with each other; and if the main artery of the limb is compressed or tied, the blood still finds its way to the parts below by this circuitous route of communication. The arteries supplying the intestines also communicate freely with each other, so that the circulation is not interrupted by the folding or bending of the parts upon each other.

The most remarkable instance of arterial anastomosis is that at the base of the brain, where the two principal arteries entering the skull from behind, namely, the right and left vertebral, unite in a single trunk, which afterward divides and sends branches forward on each side to inosculate with the two internal carotid arteries, which themselves afterward communicate with each other by a transverse anastomosis at the anterior part of the brain. Thus there is at the base a continuous vascular circle or ring, called the "circle of Willis," supplied with blood at the same time from four different arteries, the two carotids and the two vertebrals, and from which branches are sent off to the various parts of the cerebral substance. In the veins anastomosis is much more frequent, even the larger veins of the upper and lower limbs seldom continuing far in their course without giving and receiving branches of communication with adjacent veins. Thus a passage for the blood from the extremities toward the heart is constantly kept open, notwithstanding the compression to which these vessels are liable by the contraction of the muscles and accidental pressure. In the capillary blood vessels, finally, anastomosis is abundant and constant.

All the capillary blood vessels, in fact, inosculate with each other in every direction, and in such profusion as to form a network or plexus of minute vessels, with corresponding interspaces or islets included between the meshes. This provides for a continuous and uniform supply of blood to every part of the organ, and brings the blood into close contact with the substance of its tissue.