Axis (anatomy) The condition, where the dens is separated from the body of the axis, is called os odontoideum, and may cause nerve and circulation compression syndrome. On its anterior surface is an oval or nearly circular facet for articulation with that on the anterior arch of the atlas. On the back of the neck, and frequently extending on to its lateral surfaces, is a shallow groove for the transverse atlantal ligament which retains the process in position. The apex is pointed, and gives attachment to the apical odontoid ligament; below the apex the process is somewhat enlarged, and presents on either
High-altitude adaptation in humans Tibetans, Andeans and Ethiopians is indeed an observable instance of the process of natural selection in acting on favourable characters such as enhanced respiratory mechanisms in humans. Humans are naturally adapted to lowland environment where oxygen is abundant. When people from the general lowlands go to altitudes above , with atmospheric pressure 74% of normal they experience mountain sickness, which is a type of hypoxia, a clinical syndrome of severe lack of oxygen. Complications include fatigue, dizziness, breathlessness, headaches, insomnia, malaise, nausea, vomiting, body pain, loss of appetite, ear-ringing, blistering and purpling of the hands and feet, and dilated veins.