Sanguine and phlegmatic dating
Scholars of later centuries swallowed his descriptions whole—correct and incorrect—not even subjecting them to the scrutiny called for in Galen’s principle of discovery by experiment.
In actually testing animals he differentiated sensory and motor nerves, elucidated the effects of transection of the spinal cord, examined the physiological actions of the chest cavity, and proved that the heart could continue to beat without nerves.
Although he gave up surgical activities in Rome (the social pressures to avoid operations may have been too strong) his former association with surgery formed the basis for his extensive, detailed, and brilliant discussions of surgical treatment.
Wherever he went, he found anatomy repeatedly emphasized; in Smyrna, Corinth, and Alexandria he came under the influence of several famous teachers of anatomy.
Since direct dissection of the human body, which had earlier been the principle in Alexandria, was no longer practiced, Galen and other anatomists had to seek information in other ways: observing the chance exposure of organs in an injury; discovering fortuitously an abandoned corpse; dissecting animals and assuming their similarity to humans.
Because his knowledge was derived for the most part from animal (principally the Barbary ape) rather than human dissection, Galen made many mistakes, especially concerning the internal organs.
He found pharmacologic information about plants and minerals also widely available in each new region.
Furthermore, Galen was able to observe many kinds of illness, treatments, and medical philosophies, especially in Alexandria where physicians from all over the Roman world gathered to study, teach, and practice.