A scanning electron microscope study of human cerebral arteries.

Human cerebral arteries were obtained from autopsy, fixed under pressure, cut open, and tacked onto pieces of cork. For one artery the intima was partly teased away, exposing the media, and treated with a silver nitrate process. For another artery the adventitia was exposed. Both arteries were processed through graded ethanols and coated with gold paladium for the scanning electron microscope. The collagen fibers of the adventitia were approximately 5 mum in diameter and consisted of a bundle of microfilaments, each of which had a diameter of 800-1000 A (1 A = 10(-10) m). The collagen fibers were oriented parallel to the long axis of the artery. The muscle cells of the media had a diameter of 2-5 mum and were arranged circumferentially with a pitch of approximately 20 degrees. The collagen fibers of the media travel perpendicular to the muscle cells, and parallel to the long axis of the artery. The fibrillar components of the elastin in the intima had a diameter of approximately 700-1000 A and were arranged parallel to the long axis of the artery. It was postulated that the fibrillar part of the elastin was the elastic component of the elastin.     

Free‐riders promote free‐riding in a real‐world setting

A common paradigm in studies of human cooperation is that individuals mediate their investment in a public good according to the investments of others. However, most research on this topic has been conducted in artificial laboratory settings and has assumed that cooperation imposes relatively high monetary costs on players. As such, we do not yet know how humans behave when faced with non‐monetary costs of cooperating in more realistic settings. Here, we present results from a real‐world public good experiment where the monetary costs of cooperation were relaxed. We manipulated the cleanliness of a sink in a staff tea room (a public good) by adding or removing dirty kitchenware and then recorded the number of items subsequently added to the sink by staff members. We found that when the sink had no dirty items, people were less likely to dump their own used kitchenware and the public good was maintained. In contrast, people were more likely to dump items in the sink when it already contained unwashed kitchenware and increasing the number of dirty items in the sink led to an increase in the number of items that were subsequently dumped. This study shows that defection begets defection in real‐life situations, even when cooperation is relatively cheap.