In Hirschsprung disease, the migration is not complete and part of the colon lacks these nerve bodies that regulate the activity of the colon. The affected segment of the colon cannot relax and pass stool through the colon, creating an obstruction.The most accepted theory of the cause of Hirschsprung is a defect in the craniocaudal migration of neuroblasts originating from the neural crest that occurs during the first 12 weeks of gestation. Defects in the differentiation of neuroblasts into ganglion cells and accelerated ganglion cell destruction within the intestine may also contribute to the disorder.This lack of ganglion cells in the myenteric and submucosal plexus is well documented in Hirschsprungs disease. With Hirschsprungs disease, the segment lacking neurons (aganglionic) becomes constricted, causing the normal, proximal section of bowel to become distended with feces. This narrowing of the distal colon and the failure of relaxation in the aganglionic segment are thought to be caused by the lack of neurons containing nitric oxide synthase.The most cited feature is absence of ganglion cells: notably in males, 75% have none in the end of the colon (rectosigmoid) and 8% lack ganglion cells in the entire colon. The enlarged section of the bowel is found proximally, while the narrowed, aganglionic section is found distally, closer to the end of the bowel. The absence of ganglion cells results in a persistent overstimulation of nerves in the affected region, resulting in contraction.The equivalent disease in horses is lethal white syndrome. Diagnosis
Definitive diagnosis is made by suction biopsy of the distally narrowed segment.
Progressive changes occurring throughout life in the soft tissues of the face will cause more prominent facial asymmetry in older faces. Therefore, symmetrical transformation of older faces generally increases their attractiveness while symmetrical transformation in young adults and children will decrease their attractiveness. Personality and Big Five personality traits
Research indicates that facial symmetry is correlated with the big-five model of personality. The five factors are:
Openness to experience (inventive/curious vs. consistent/cautious)
Conscientiousness (efficient/organized vs. easy-going/careless)
Extraversion (outgoing/energetic vs. solitary/reserved)
Agreeableness (friendly/compassionate vs. challenging/detached)
Neuroticism (sensitive/nervous vs. secure/confident)A consistent finding is that facial symmetry is positively correlated with extraversion, indicating that individuals with more symmetric faces are also more extroverted. More symmetrical faces are also judged to be lower on neuroticism but higher on conscientiousness and agreeableness (asymmetrical faces were rated as less agreeable than normal ones, but the more symmetrical were again rated as somewhat less agreeable than the normal). More symmetrical faces are also more likely to have more desirable social attributes assigned to them, such as sociable, intelligent or lively.However, the relationship of facial symmetry and the big-five personality model remains somewhat unclear with regard to neuroticism, openness, agreeableness and conscientiousness. Openness and agreeableness appear to be significantly negatively related to facial symmetry, while neuroticism and conscientiousness do not seem to be linked to facial symmetry. With respect to trustworthiness it has been found that the facial muscles become imbalanced when lying. Evolution and sexual selection
Sexual selection is a theoretical construct within evolution theory. According to sexual selection, mate choice can have profound influence on the preferred features.