The corpus callosum is a structure of the mammalian brain in the longitudinal fissure that connects the left and right cerebral hemispheres. It is the largest white matter structure in the brain, consisting of 200-250 million contralateralaxonal projections. It is a wide, flat bundle of axons beneath the cortex. Much of the inter-hemispheric communication in the brain is conducted across the corpus callosum.
Monotremes and marsupials do not have a corpus callosum.
The posterior portion of the corpus callosum is called the splenium; the anterior is called the genu (or "knee"); between the two is the 'body.'
The most anterior part is the rostrum.
There are disputed claims about the difference of the size of the human corpus callosum
in men and women and the relationship of any such differences to gender differences in human behaviour and cognition.
RB Bean, a Philadelphia anatomist, suggested in 1906 that the "exceptional size of the corpus callosum may mean exceptional intellectual activity" and claimed differences in size between males and females and between races, although these were refuted by Franklin Mall, the director of his own laboratory.
Of much more substantial popular impact was a 1982 Science article claiming to be the first report of a reliable sex difference in human brain morphology, and arguing for relevance to cognitive gender differences. This paper appears to be the source of a large number of lay explanations of perceived male-female difference in behaviour: For example Time magazine was reported to state in 1992 that the corpus callosum is "Often wider in the brains of women than in those of men, it may allow for greater cross-talk between the hemispheres—possibly the basis for woman’s intuition." It has also been used, for example, as the explanation of an increased single-task orientation of male, relative to female, learners; a smaller male corpus is said to make it harder for the left and right sides of the brain to work together and to explain a greater feminine ability to multitask.
There is scientific dispute not only about the implications of anatomical difference, but whether such a
difference actually exists. A substantial review paper performed a meta-analysis of 49 studies and found, contrary to de Lacoste-Utamsing and Holloway, that males have a larger corpus callosum, a relationship that is true whether or not account is taken of larger male brain size. Bishop and Wahlstein found that "the widespread belief that women have a larger splenium than men and consequently think differently is untenable." However, more recent studies using new techniques revealed morphological sex differences in human corpus callosum. Whether, and to what extent, these morphological differences are associated with behavioural and cognitive differences between males and females remains unclear.
The corpus callosum has been reported to be significantly larger in musicians than non-musicians, and to be slightly larger in left-handed people than right-handed people.
The symptoms of refractory epilepsy can be reduced by cutting the corpus callosum in an operation known as a corpus callosotomy. 
^ ab Bishop KM, Wahlsten D (1997). "Sex differences in the human corpus callosum: myth or reality?". Neurosci Biobehav Rev21 (5): 581–601. PMID 9353793.
^ de Lacoste-Utamsing, C., Holloway, R. L. "Sexual dimorphism in the human corpus callosum." Science, 216, 1431–1432, 1982.
^ C Gorman (20 January 1992). "Sizing up the sexes". Time: 36-43.
^ Dubb A, Gur R, Avants B, Gee J (2003). "Characterization of sexual dimorphism in the human corpus callosum". Neuroimage20 (1): 512–9. PMID 14527611.
^ Shin YW, Kim DJ, Ha TH, et al (2005). "Sex differences in the human corpus callosum: diffusion tensor imaging study". Neuroreport16 (8): 795–8. PMID 15891572.
^ Levitin, Daniel J. "This is Your Brain on Music", '
^ Driesen, Naomi R.; Naftali Raz (1995). "The influence of sex, age, and handedness on corpus callosum morphology: A meta-analysis". Psychobiology23 (3): 240–247.
^ Clarke DF, Wheless JW, Chacon MM, et al (2007). "Corpus callosotomy: a palliative therapeutic technique may help identify resectable epileptogenic foci". Seizure16 (6): 545–53. doi:10.1016/j.seizure.2007.04.004. PMID 17521926.