In this owl, rescued from the roadside, there seems to be a black slug causing haemorrhage in the back of the eye. What are we seeing?
The black slug is the pecten, a sheet of vascularised tissue emminating from the choroid, acting to provide oxygen and nutrition to the otherwise avascular retina as it waves from side to side as the eye is moved by small saccadic oscillations, as first beautifully described by John Pettigrew in his seminal paper ‘Saccadic oscillations facilitate ocular perfusion from the avian pecten.’ Nature. 1990 343:362-3. Here’s a more normal pecten – the previous one was persistently haemorrhaging after blunt trauma.