Chemoarchitecture was used to delineate more accurately the human reticular formation and some other nuclei of the human brainstem. These observations make it easier for scientists to create models of human pathology in rats and other experimental animals. With better knowledge of which brainstem structures correspond in rat and human scientists inspired by human considerations can more readily test their hypotheses on rats and then extrapolate their observations to humans.

The keystone to the organizational plan of the medullary reticular formation is the intermediate reticular zone (IRt).

We acknowledge the radial arrangement of the human medulla with reference to the fourth ventricle (as King, 1980, proposed for the cat) is more tenable than the "quilt" pattern proposed by Olszewski and Baxter (1954). Thus, it appears that the human medulla is organised roughly in columns, commencing with a special afferent zone (vestibular nuclei) dorsolaterally and terminating in a general motor efferent zone ventromedially (hypoglossal). Intervening in a dorsal to ventral sequence are the somatic afferent column (spinal nucleus of the trigeminal), the visceral afferent column (solitary nucleus and the dorsolateral slab of the intermediate reticular zone), and the visceral or branchial efferent column (dorsal motor nucleus of vagus, ambiguus, and the ventromedial part of IRt). A scheme of organisation along these lines, was suggested by Herrick (1922) for the cranial nerve nuclei and is now popularly used in many neuro-anatomical text books.

Conclusion
The most significant conclusion of this overview is a glaring structural similarity of brainstem across species reflected by an impressive number of homologies recognised between the brainstem of the human and that of other animals. What is more surprising is that the human brainstem is also highly homologous with the brainstem of the chick (Puelles, Martinez-de-la-Torre, Martinez, Watson and Paxinos, in press). It seems that this part of the brain is highly conserved in the vertebrates that arose approximately 400 million years ago.