Sentence:  As retinal images are conceded to be an integral function of the <WSD>brain</WSD> 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:  that part of the central nervous system that includes all the higher nervous centers; enclosed within the skull; continuous with the spinal cord