These factors can be grouped into four categories: (1) disordered motility or movement of the small bowel or anatomical changes that lead to stasis, (2) disorders in the immune system, (3) interference with the production of proteolytic enzymes, gastric acid, or bile, and (4) conditions that cause more bacteria from the colon to enter the small bowel. In some people, methanogens may reside in the oral cavity, as evidenced by reductions in breath methane levels following mouthwash with chlorhexidine. This may affect results from hydrogen-methane breath testing. Absence or impairment of the migrating motor complex (MMC), a cyclical motility pattern in the small intestine, and phase III of the MMC in particular, is associated with the development of SIBO. Problems with motility may either be diffuse, or localized to particular areas. MMC impairment may be a result of post-infectious irritable bowel syndrome, drug use, or intestinal pseudo-obstruction among other causes. There is an overlap in findings between tropical sprue, post-infectious irritable bowel syndrome and small intestinal bacterial overgrowth in the pathophysiology of the three conditions and also SIBO can similarly sometimes be triggered by an acute gastrointestinal infection. As of 2020, there is still controversy about the role of SIBO in the pathogenesis of common functional symptoms such as those considered to be components of irritable bowel syndrome. Diseases like scleroderma cause diffuse slowing of the bowel, leading to increased bacterial concentrations. More commonly, the small bowel may have anatomical problems, such as out-pouchings known as diverticula that can cause bacteria to accumulate.
Anatomy
The human skull is anatomically divided into two parts: the neurocranium, formed by eight cranial bones that houses and protect the brain—and the facial skeleton (viscerocranium) composed of fourteen bones, not including the three ossicles of the inner ear. The term skull fracture typically means fractures to the neurocranium, while fractures of the facial portion of the skull are facial fractures, or if the jaw is fractured, a mandibular fracture.The eight cranial bones  are separated by sutures : one frontal bone, two parietal bones, two temporal bones, one occipital bone, one sphenoid bone, and one ethmoid bone.The bones of the skull are in three layers: the hard compact layer of the external table (lamina externa), the diploë (a spongy layer of red bone marrow in the middle, and the compact layer of the inner table (Lamina interna).Skull thickness is variable, depending on location. Thus the traumatic impact required to cause a fracture depends on the impact site. The skull is thick at the glabella, the external occipital protuberance, the mastoid processes, and the external angular process of the frontal bone.