In the Langendorff preparation, the heart is removed from the animal's body, severing the blood vessels; it is then perfused in a reverse fashion via the aorta, usually with a nutrient rich, oxygenated solution (e.g. Krebs–Henseleit solution or Tyrode's solution). The backwards pressure causes the aortic valve to shut, forcing the solution into the coronary vessels, which normally supply the heart tissue with blood. This feeds nutrients and oxygen to the cardiac muscle, allowing it to continue beating for several hours after its removal from the animal. This is a useful preparation because it allows the addition of drugs (via the perfusate) and observation of their effect on the heart without the complications involved with in vivo experimentation, such as neuronal and hormonal effects from living animal. This preparation also allows the organ to be digested into individual cells by adding collagenase to the perfusate. This can be done before the experiment as a technique for cell harvesting, or after the experiment to measure its effects at the cellular level.