Sentence:  As retinal images are conceded to be an integral <WSD>function</WSD> of the brain it seems logical to suppose that the nerves , between the inner brain and the eyes , carry the direct drive for cooperation from the various brain centers - rather than to theorize on the transmission of an image which is already in required location . 
Meaning of the word marked with <WSD> tags:  the actions and activities assigned to or required or expected of a person or group