Blood vessels. Coloured scanning electron micrograph (SEM) of a resin cast of the blood vessels that surround the tear (lachrymal) glands. This network of vessels infiltrate the glands' tissues, supplying them with blood. Gases and nutrients are exchanged between the blood and surrounding tissue via the permeable walls of capillaries. The lachrymal glands are found above and behind each eye, behind the eyelid. When the eyelid blinks, lachrymal fluid is pulled from the glands, washing the eye. Strong emotion causes the production of tears. The cast was made by injecting resin into the blood vessels, followed by chemical digestion of the surrounding tissue.