Hales began his work on animal physiology with William Stuckeley while in Cambridge, although much of it was published only after "Vegetable Staticks" appeared. Hales and Stuckeley performed a wide range of studies including making casts of the trachea and bronchial trees of dogs using molten lead and measuring the water lost due to breathing. Most famously, Hales made measurements of blood pressure in several animal species by inserting fine tubes into arteries and measuring the height to which the column of blood rose. Hales also described the effects of hemorrhage and hemorrhagic shock by progressive exsanguination of animals and accompanying measurement of blood pressure. In a horse he observed that as death approached "the Mare fell into cold and clammy sweats". In addition, Hales took wax casts of the ventricle of the heart and estimated how much blood was pumped by the heart; correctly described the roles of the mitral valve and aortic valve during systole and diastole; explained the pulsations of arteries in terms of their elasticity and attributed the resistance to blood flow to friction due to the passage of blood through small blood vessels.