If something is situated between, but not within, the parts of a tissue, it is said to be

It is said to be INTERSTITIAL. 
Interstitial refers to location of a thing, when a substance is located between two things that are typically closely spaced. The location where the thing is situated is called interstice. The word is used in medical terminology to specify the location of a thing .

Twenty one miles per hour

The answer to this will be from your lab notes/procedure. it would be best to ask another student who participated in the lab as we do not know what was done, what enzyme or if the temperature is celsius or fahrenheit. if i were to guess though i'd say the rate stopped due to enzyme denturing at high temperature