Galen of Bergama authors several hundred works (eighty of which are extant) on medicine and healing and, through experiments upon animals, founds the science of physiology.

A Greek philosopher and physician from Asia Minor who serves the emperor Marcus Aurelius, strongly influenced by Aristotle and Hippocrates’ beliefs in vital essences, Galen, though possessing practical knowledge of the circulatory system, postulates the transport of blood through vessels to the skin, where it becomes flesh.

Maintaining that knowledge of human anatomy is fundamental for a physician, he obtains anatomical facts from the dissection of animals such as pigs, dogs, and goats.

Prevented by law and custom from working with human bodies, Galen’s work, although notable, incorporates many errors when applied to human anatomy.

He nevertheless identifies numerous muscles for the first time and demonstrates the importance of the spinal cord, recording the resulting paralysis when the cord was cut at different levels.

The first to consider the pulse a diagnostic aid, he also explains the function of many nerves, discovers the sympathetic nervous system, and details almost all the structures of the brain visible to the naked eye.

His physiological theories include concepts of blood formation, digestion, and nerve function (but his insistence that tiny pores exist in the heart through which blood passes from the right to the left ventricle is an error that will be accepted throughout Europe for more than a thousand years).

Galen is an enthusiastic advocate of the virtues of opium; his books become the supreme authority on the subject for hundreds of years.

Galen had gone to Rome in 162 and had made his mark as a practicing physician.

His impatience has brought him into conflict with other doctors and he feels menaced by them.

His demonstrations here had antagonized the less able and original physicians in the city.

They plotted against him and he, fearing he might be driven away or poisoned, had left the city.