Trachea Diffusion of oxygen and carbon dioxide takes place across the walls of the smallest tubes, called tracheoles, which penetrate tissues and even indent individual cells. Gas may be conducted through the respiratory system by means of active ventilation or passive diffusion. Unlike vertebrates, insects do not generally carry oxygen in their haemolymph. This is one of the factors that may limit their size. A tracheal tube may contain ridge-like circumferential rings of taenidia in various geometries such as loops or helices. Taenidia provide strength and flexibility to the trachea. In the head, thorax, or abdomen, tracheae may also be connected
The Description of the Human Body The Description of the Human Body The Description of the Human Body () is an unfinished treatise written in 1647 by René Descartes (1596-1650). Descartes felt knowing oneself was particularly useful. This for him included medical knowledge. He hoped to cure and prevent disease, even to slow down aging. René Descartes believed the soul caused conscious thought. The body caused automatic functions like the beating of the heart and digestion he felt. The body was necessary for voluntary movement as well as the will. However, he believed the power to move the body was wrongly imagined to come from the