Galen (131 - 201 AD) a physician to the Roman gladiators. He didn't dissect human cadavors but Galen's great powers of observation allowed him to study, classify, and record anatomic and pathologic observations. Some of his observations, like the crab-like growth of cancers, were correct.
Leonardo da Vinci (1452 -1519) was one of the first to dissect humans in the quest for answers about the human form.
Andreas Vesalius (1514 - 1564) dissected humans and published a book De Humani Corporis Fabrica, 1543. His drawings of the human skeleton are especially noted for their detail and artestry.
More images and pages of his book can be seen here.
Antonio Benivieni (1443 - 1502) is credited for performing the first autopsies on humans to determine the cause of death. Not much is known about Benivieni's life.
Anatomy as a subject of study became acceptible in the 17th century.
Marie-Francois-Xavier Bichat (1771 - 1802), a French physcian was one of the first 'modern' pathologists and is considered by many to be the father of histology and pathology.
John Hunter (1728 - 1793), an Englishman who devised a method for preserving diseased tissues so they could be studied in the future.
Carl Rokitansky (1804 – 1878) is said to have performed over 20,000 autopsies and oversaw the performance of 60,000 more. He made important observations concerning multiple sclerosis.
Rudolf Virchow (1821 - 1905), used the microscope to learn that diseases arise from alterations within tissues and cells.
Julius Cohnheim (1839–1884), Virchow's student, devised experiments to study inflammation as it happened rather than after the tissue was dead. The Father of Experimental Pathology.
Some information about these pathologists was obtained here.