<P> Al - Kindi warned against alchemists attempting the transmutation of simple, base metals into precious ones like gold in the ninth century . </P> <P> Al - Biruni (973 - 1048) estimated the radius of the earth as 6339.6 km (modern value is c. 6,371 km), the best estimate at that time . </P> <P> In the cardiovascular system, Ibn al - Nafis in his Commentary on Anatomy in Avicenna's Canon was the first known scholar to contradict the contention of the Galen School that blood could pass between the ventricles in the heart through the cardiac inter-ventricular septum that separates them, saying that there is no passage between the ventricles at this point . Instead, he correctly argued that all the blood that reached the left ventricle did so after passing through the lung . He also stated that there must be small communications, or pores, between the pulmonary artery and pulmonary vein, a prediction that preceded the discovery of the pulmonary capillaries of Marcello Malpighi by 400 years . The Commentary was rediscovered in the twentieth century in the Prussian State Library in Berlin; whether its view of the pulmonary circulation influenced scientists such as Michael Servetus is unclear . </P> <P> In the nervous system, Rhazes stated that nerves had motor or sensory functions, describing 7 cranial and 31 spinal cord nerves . He assigned a numerical order to the cranial nerves from the optic to the hypoglossal nerves . He classified the spinal nerves into 8 cervical, 12 thoracic, 5 lumbar, 3 sacral, and 3 coccygeal nerves . He used this to link clinical signs of injury to the corresponding location of lesions in the nervous system . </P>

Which two caliphates are most closely affiliated with the islamic golden age