Species Differences
The corpus callosum is found only in placental mammals (the eutherians), while it is absent in monotremes and marsupials, as well as other vertebrates such as birds, reptiles, amphibians and fish (other groups do have other brain structures that allow for communication between the two hemispheres, such as the anterior commissure, which serves as the primary mode of interhemispheric communication in marsupials, and which carries all the commissural fibers arising from the neocortex(also known as the neopallium), whereas in placental mammals the anterior commissure carries only some of these fibers). In primates, the speed of nerve transmission depends on its degree of myelination, or lipid coating. This is reflected by the diameter of the nerve axon. In most primates, axonal diameter increases in proportion to brain size to compensate for the increased distance to travel for neural impulse transmission. This allows the brain to coordinate sensory and motor impulses. However, the scaling of overall brain size and increased myelination has not occurred between chimpanzees and humans. This has resulted in the human corpus callosum's requiring double the time for interhemispheric communication as a macaque's.
The fibrous bundle that the corpus callosum appears as, can and does increase to such an extent in humans that it encroaches upon and wedges apart the hippocampal structures.
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