The blue arrow refers to letter B. If you are not too familiar with Foramen spinosum, this is one of the foramina that can be found in the skull. This is located at the base area of the skull. This is very important for doctors who are going to do neurosurgery because this can be their guide in reaching the other areas of the brain.
Do take note that the different foramina that are available can hold various structures, nerves, veins, and so much more. Without these, it can be hard for the brain to function. The foramen ovale is also located near this area and this is a part that should close all the way to ensure that everything is working well.