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COMPARATIVE ANATOMY HOME	COMPARATIVE ANATOMY TABLE OF CONTENTS		OBL HOME	OBL REFERENCES
	MIDBRAIN
In humans, the midbrain is an important region for a diversity of types of processing ranging from muscle contraction to vision and hearing. It is less prominent in mammals than other vertebrates since the development of the cerebrum has adopted some of the functions (such as association) which are performed in the midbrains of lower vertebrates. The midbrain is composed of three general regions: the tectum, the tegmentum, and the isthmus. Hagfish possess a nucleus of the superior raphe, an interpeduncular nucleus in the isthmus region of the midbrain, and distinct nuclei in the tegmentum region. (Butler, 1996, p. 207, 213-6). The retina projects to the optic tectum in hagfish as in gnathostomes (Butler, 1996, p. 244) and a prominent choroid plexus is present in the midbrain of jawless fish (Weichert, 1970, p.621). In lampreys, the midbrain contains the nuclei of cranial nerves III and IV (the latter of which departs the brain dorsally, as in gnathostomes) (Hardisty, p. 311). Lampreys possess an optic tectum and a torus semicirularis (homologous to the inferior colliculus of mammals) (Butler, 1996, p. 229). Primitively, the tectum was the primary site for visual processing. In lampreys, only a few optic fibers travel to the geniculate primordium of the thalamus and the forebrain does not respond to visual stimuli (Hardisty, 321-6). Lampreys possess a homolog of the midbrain-hindbrain boundary and the genes expressed in this area are similar to those found in gnathostomes (Murakami, 2005).
HAGFISH		LAMPREY
Gnathostome midbrains share a number of regions such as a cuneiform nucleus, intercollicular nucleus, red nucleus, locus coeruleus, and a substantia nigra. (Butler, 1996, p. 207-17). The tectum is better developed in gnathostomes and there is an increase in basal region of midbrain. The midbrain of gnathostomes affects body position through connections with cerebellum and medulla and the primitive ependymal region of the midbrain is absent (Ariens, p.1191-3 ). In lower vertebrates, there is an optic ventricle in the superior colliculi which fuses across the midline. (Weichert, 1970, p.621). Gene expression and neural structures are comparable between the lamprey and gnathostome midbrains and diencephalons (Murakami, 2005).

In bony fish and tetrapods, the red nucleus forms part of a rubrospinal tract and a nucleus isthmus exists (Butler, 1996, p. 207-17). A number of commissures exist in the midbrain and the tori semicirculares are better developed in bony fish and tetrapods (Ariens, p.1195).
PERCH		LUNGFISH
In tetrapods, reticular regions exist in the midbrain and the optic tectum covers tori semicirculares. Afferent fibers lead to the optic tectum even in blind amphibians (Ariens, p.1196).
FROG

Amniotes possess a pedunculopontine nucleus (Butler, 1996, p. 216-7) and a homolog of the large-celled region of the nucleus ruber (Ariens, p.1197). Ipsilateral sensory projections from midbrain travel to the pallium (Butler, 1995). In most amniotes, the lateral line component of the torus semicircularis has been lost (monotremes are an exception) (Butler, 1996, p. 231.) In amniotes, the ventral tegmental area and substantia nigra project to the striatum with dopaminergic neurons (Medina, 1995).

TURTLE


In mammals, the primitive optic tectum forms the superior colliculi and the primitive tori semicirculares form the inferior colliculi (Ariens, p.1202). The inferior colliculus is exposed on the surface of the midbrain rather than being deep to the optic tectum (Webster, 1974, p. 259) and a fissure splits superior and inferior colliculi (Weichert, 1970, p. 621). Only mammals possess cerebral peduncles. (Webster, 1974, p. 259) There are increased connections between the superior colliculus and diencephalon, an increase in the medial longitudinal fasciculus, the red nucleus is more prominent, and a pedunculo-tegmental system exists (Ariens, p.1202). The size difference between superior and inferior colliculi (the superior colliculus is the larger of the two) is greater in humans than in Old World monkeys. (Hartman, 1933, p. 284). In most placental mammals, the expansion of the cerebrum and cerebellum result in the covering the midbrain, ending the “midbrain exposure” which is typical of primitive mammals and non-mammals.
OPOSSUM

CAT
SHEEP
		PIG
MONKEY

HUMAN