Sinoatrial node therefore cause ischaemia and cell death in the SA node. This can disrupt the electrical pacemaker function of the SA node, and can result in sick sinus syndrome. If the SA node does not function, or the impulse generated in the SA node is blocked before it travels down the electrical conduction system, a group of cells further down the heart will become its pacemaker. The sinoatrial node was first discovered by a young medical student, Martin Flack, in the heart of a mole, whilst his mentor, Sir Arthur Keith, was on a bicycle ride with his wife. They made
Lymph node range in size from a few millimeters to about 1–2 cm long. Each lymph node is surrounded by a fibrous capsule, which extends inside the lymph node to form trabeculae. The substance of the lymph node is divided into the outer cortex and the inner medulla. The cortex is continuous around the medulla except where the medulla comes into direct contact with the hilum. Thin reticular fibers of reticular connective tissue, and elastin form a supporting meshwork called a "reticulin" inside the node. B cells are mainly found in the outer (superficial) cortex where they are clustered together as follicular