Hall's experiments on the physiological effects of therapeutic bleeding provided compelling evidence that bleeding could cause significant harm and even death. Digital facsimile from wellcomecollection.org at this link.

Marshall Hall clearly distinguished arterioles and venules from capillaries, and he described arteriovenous shunts.

“Hall showed that reflex activity could be distinguished from other types of movement, that it produced what today we call ‘muscle tone,’ that it included sneezing coughing, and vomiting, and that it could be influenced by disease. The discovery of these characteristics, and the general formulation of the reflex concept, remain Hall’s outstanding contributions… [Hall and Johannes Müller] were able to rely entirely upon their experimental findings, and, unlike their predecessors, could discount the intervention of the soul and so exclude the metaphysical miasma that had clouded much of the previous work on the subject of reflex activity” (Clarke and O’Malley, 1996, p. 347). 

Hall was the first to suggest that the paroxysmal nervous discharges in epilepsy were produced by the spinal nervous system, first to notice the connection of anemia with epilepsy, and first to deduce that epilepsy was produced by anemia of the medulla.

Marshall Hall’s method of artificial respiration.