An agent that is smaller than a virus causes scrapie. It is either a protein called a prion or it is another substance closely associated with the prion protein. Scientists still have a lot to learn about the scrapie agent and exactly how it works.
Tissue valves are made with tissues from porcine (pig) heart valves or bovine (cow) cardiac tissue because they function like human heart valves. Once the tissue is removed from the animal, it is chemically treated to preserve the tissue and prevent immulogic reactions once it is placed in a patient.
This valve is made up of the following structures: 1  Velum (also called soft palate) 2  Lateral pharyngeal walls â the side walls of the throat. 3  Posterior pharyngeal wall â the back wall of the throat.