Simplify the following text for a Grade 10 native English-speaking person.
Original: Since 1861 according to the constitution of 28 September 1860 the state has been governed by the ten-member Senate , which had been called council ( in the German language of that time : Rath ) before that time .
Simplified: The constitution of 28 September 1860 says that the state is to be governed by the ten-member Senate headed by the First Mayor as the President of the Senate .