Page:EB1911 - Volume 19.djvu/455

 the myelin sheath and ending in an increase of the supporting neuroglia tissue at the expense of the true nervous tissue. Tumours and new growths in the central and peripheral nervous systems may be primary or secondary: the former arise in the supporting, enclosing or nutrient tissue elements; the latter are metastatic deposits from tumours originating elsewhere. Tumours may be single or multiple, the special symptoms occasioned depending upon the seat of the tumour and whether it destroys or only irritates the adjacent nervous tissue. Tumours situated within the cranial cavity cause general symptoms, namely, optic neuritis, severe headache and vomiting; these symptoms, which are caused by increased intracranial pressure, are more severe in rapidly-growing vascular tumours, even though small, than in large slow-growing tumours.