a vascular network formed as a result of the one-time division of the original blood vessel into capillary-like branches that subsequently reunite into a single vessel.

One of the many anachronisms in terminology, the term was coined by Galen, who discovered the division of the internal right and left carotid arteries in animals into numerous small arterial branches, which then rejoined between the internal base of the skull (in the body of the sphenoid bone and the upper part of the clivus) and the dura mater of the brain. The unusual feature of this vascular network of vessels is that all the smaller vessels eventually unite into a single vessel that continues further under the same name—the internal carotid artery—and that acts as the source for the capillary network of the corresponding hemispheres of the brain. Commentators on Galen’s works believed that the life spirit (spiritus vitalis) was transformed into the animal spirit (spiritus animalis) in the rete mirabile and was then distributed throughout the entire body from the brain along the nerves, as if through pipes.

In the modern literature, the term “rete mirabile” sometimes refers to the glomerular capillaries of the kidneys that join blood vessels bringing blood into the glomeruli and then taking it out (rete mirabilis artereosum) and to the sinusoids in the liver, which connect the branches of the portal vein with the roots of the hepatic veins (rete mirabilis venosum).

The rete mirabile in the wall of the swim bladder of fishes is composed of the finest precapillary arterioles, through which gas passes into the swim bladder from the blood cells. The function of the rete mirabilis is to retard the flow of blood in certain parts of the circulatory system.