Herophilos was born in Chalcedon in Asia Minor (now Kadiköy, Turkey), c. 335 BC. Not much is known about his early life save his moving to Alexandria at a fairly young age to begin his schooling.
As an adult Herophilos was a teacher, and an author of at least nine texts ranging from his book titled, On Pulses, which explored the flow of blood from the heart through the arteries, to his book titled Midwifery, which discussed duration and phases of childbirth. In Alexandria, he practiced dissections, often publicly so that he could explain what he was doing to those who were fascinated. Erasistratus was his contemporary. Together, they worked at a medical school in Alexandria that is said to have drawn people from all over the ancient world due to Herophilos' fame.
His works are lost but were much quoted by Galen in the second century AD. Herophilos was the first scientist to systematically perform scientific dissections of human cadavers. Dissections of human cadavers were banned in most places at the time, except for Alexandria. Celsus in De Medicina and the church leader Tertullian state that he vivisected at least 600 live prisoners.
After the death of Herophilos in 280 BC, his anatomical findings lived on in the works of other important physicians, notably Galen. Even though dissections were performed in the following centuries and medieval times, only a few insights were added. Dissecting with the purpose to gain knowledge about human anatomy started again in early modern times (Vesalius), more than 1600 years after Herophilos' death.
He emphasised the use of experimental method in medicine, for he considered it essential to found knowledge on empirical bases.
Conventional medicine of the time revolved around the theory of the four humors in which an imbalance between bile, black bile, phlegm, and blood led to sickness or disease. Veins were believed to be filled with blood and a mixture of air and water. Through dissections, Herophilus was able to deduce that veins only carried blood. After studying the flow of blood, he was able to differentiate between arteries and veins. He noticed that as blood flowed through arteries, they pulsed or rhythmically throbbed. He worked out standards for measuring a pulse and could use these standards to aid him in diagnosing sicknesses or diseases. To measure this pulse, he made use of a water clock.
His work on blood and its movements led him to study and analyse the brain. He proposed that the brain housed the intellect rather than the heart. He was the first person to differentiate between the cerebrum and the cerebellum and to place individual importance on each portion. He looked more in depth into the network of nerves located in the cranium. He described the optic nerve and the oculomotor nerve for sight and eye movement. Through his dissection of the eye, he discovered the different sections and layers of the eye: the cornea, the retina, the iris, and the choroid also known as the choroid coat. Further study of the cranium led him to describe the calamus scriptorius which he believed was the seat of the human soul. Analysis of the nerves in the cranium allowed him to differentiate between nerves and blood vessels and to discover the differences between motor and sensory nerves. He believed that the sensory and motor nerves shot out from the brain and that the neural transmissions occurred by means of pneuma. Part of his belief system regarding the human body involved the pneuma, which he believed was a substance that flowed through the arteries along with the blood. Playing off of medical beliefs at the time, Herophilos stated that diseases occurred when an excess of one of the four humors impeded the pneuma from reaching the brain.
Herophilos also introduced many of the scientific terms used to this day to describe anatomical phenomena. He was among the first to introduce the notion of conventional terminology, as opposed to use of "natural names", using terms he created to describe the objects of study, naming them for the first time. A confluence of sinuses in the skull was originally named torcular Herophili after him. Torcular is a Latin translation of Herophilos' label, ληνός - lenos, 'wine vat' or 'wine press'. He also named the duodenum, which is part of the small intestine. Herophilos is credited with learning extensively about the physiology of the reproductive system. In his book entitled Midwifery, he discussed phases and duration of pregnancy as well as causes for difficult childbirth. The aim of this work was to help midwives and other doctors of the time more fully understand the process of procreation and pregnancy. He is credited with the discovery of the ovum. Other areas of his anatomical study include the liver, the pancreas, and the alimentary tract, as well as the salivary glands and genitalia.
Herophilos believed that exercise and a healthy diet were integral to the bodily health of an individual. Herophilos once said that "when health is absent, wisdom cannot reveal itself, art cannot become manifest, strength cannot be exerted, wealth is useless, and reason is powerless".