Ridges of tissue on the surface of the cerebral hemispheres are called

The undulations on the cerebral cortex (the outer layer of the brain) are called gyri (ridges) and sulci (gorges). Also called the grey matter, the cerebral cortex is the core of brain functioning region with approximately 20 billion neurons. 
It is believed that the cerebral cortex is highly folded to accommodate many neurons.

Balkan nationalism increased tensions between austria and russia