The brain shift that occurs during neurosurgery is a serious matter impacting the patient safety as well as the invasiveness of the brain surgery. Model driven compensation is a realistic and efficient solution to solve this problem. However, its key issue is the lack of reliable and easily obtainable patient specific mechanical characteristics of the brain which, according to clinicians experience, can vary in a wide range. We designed an aspiration device that is able to meet the very severe sterilization and handling process imposed during surgery, and especially neurosurgery. The device, which has no electronic component, is simple, light and can be considered as an ancillary instrument. The deformation of the aspired tissue is imaged via mirror using an external camera. This paper describes the experimental setup as well as its use during a neurosurgery. The experimental data were used to calibrate a continuous model. We show that we were able to extract in vivo a constitutive law of the brain elasticity: for the first time, measurements are carried out per-operatively on the patient, just before the resection of brain parenchyma. We mostly want to disclose here the results of a delicate experiment and provide for the first time in-vivo data on human brain elasticity. The results point out the softness as well as the highly non linear behavior of the brain tissues