Bird This pericardial sac is filled with a serous fluid for lubrication. The heart itself is divided into a right and left half, each with an atrium and ventricle. The atrium and ventricles of each side are separated by atrioventricular valves which prevent back flow from one chamber to the next during contraction. Being myogenic, the heart's pace is maintained by pacemaker cells found in the sinoatrial node, located on the right atrium. The sinoatrial node uses calcium to cause a depolarising signal transduction pathway from the atrium through right and left atrioventricular bundle which communicates contraction to the ventricles. The
Lymph node and fixed macrophages. The subcapsular sinus has clinical importance as it is the most likely location where the earliest manifestations of a metastatic carcinoma in a lymph node would be found. The cortex of the lymph node is the outer portion of the node, underneath the capsule and the subcapsular sinus. It has an outer superficial part and a deeper part known as the paracortex. The subcapsular sinus drains to the trabecular sinuses, and then the lymph flows into the medullary sinuses. The outer cortex consists mainly of the B cells arranged as follicles, which may develop a germinal center