Answer
They followed his work religiously and did not bother to question his conclusions and discoveries. Medicine is a science of discovery and Galen's advice was to never stop discovering and stagnate on one conclusion, it is important in Anatomy & Physiology specifically, because most discoveries are never certain and we have to use our own logic and mind to conclude why our anatomy is the way it is and sometimes go against conventional thinking.
Work Step by Step
Galen told his followers to make their own discussions, conclusions and hypothesis, not to only rely on his and others understanding. I came to this conclusion because medicine is constantly making breakthroughs, especially in the 21st century and if we look at home many discoveries are being made that go against the conventional way of thinking you will see that Galen was speaking the truth. To apply this logic of discovery to Anatomy & Physiology we have to imagine how medicine was in the 19th century and compare it to how it is now
