Cataract surgery is the removal of the natural lens of the eye (also called "crystalline lens") that has developed an opacification, which is referred to as a cataract. Metabolic changes of the crystalline lens fibers over time lead to the development of the cataract and loss of transparency, causing impairment or loss of vision. Cataract surgery was known to the physician Sushruta in the 6th century BCE, and was performed with a special tool called the jabamukhi salaka, a curved needle used to loosen the obstructing phlegm and push it out of the field of vision. The eye would later be soaked with warm butter and then bandaged. Sushruta claimed success with this method but cautioned that this procedure should be performed only when absolutely necessary. This method may have been brought to the West by Greek travelers from India and the Middle East. The removal of cataract by surgery was also introduced into China from India.