In the healthy heart, the electrical impulses that cause contraction of the cardiac muscle are initiated in the:

Letterhead contains a(n) ________, which is a form of artwork that indicates the type of practice or another item of significance

Fill in the blank(s) with correct word

Iodine is an example of a(n) ________, which is a substance that intensifies or deepens the response of a specimen to a stain. 

Fill in the blank(s) with the appropriate word(s).