a cancerous growth or tumour composed of cells derived from neuroglial tissue, the material that supports and protects nerve cells. Gliomas may form in the retina of the eye, in the brain, in the heart, in the myelin sheaths of nerves (neurilemmoma), and occasionally in the sinus, where the tumour has been derived from the olfactory bulb of the brain. Gliomas can occur almost anywhere in connection with nerve tissue and may appear with other cancerous tissue, such as that of blood vessels, bone, or cartilage. They are of variable malignancy.