. Biology and human life. Biology. l62 BIOLOGY AND HUMAN LIFE its oxygen from its immediate neighbor- hood, and (2) discharges its carbon dioxid and other products of oxidation into its immediate surroundings. 136. Breathing in man. Breathing, or respiration, means a process of gas ex- change—taking in oxygen and giving off carbon dioxid. It makes oxidation pos- sible. In man (as, in fact, in all back- boned animals except fishes and the young of amphibians) air is taken from the outside into the lungs (soft bags sus- pended in the thorax, or chest cavity), and carbon dioxid is discharged to t
. Biology and human life. Biology. l62 BIOLOGY AND HUMAN LIFE its oxygen from its immediate neighbor- hood, and (2) discharges its carbon dioxid and other products of oxidation into its immediate surroundings. 136. Breathing in man. Breathing, or respiration, means a process of gas ex- change—taking in oxygen and giving off carbon dioxid. It makes oxidation pos- sible. In man (as, in fact, in all back- boned animals except fishes and the young of amphibians) air is taken from the outside into the lungs (soft bags sus- pended in the thorax, or chest cavity), and carbon dioxid is discharged to the exterior from the same organs (see Fig. 83). The lungs consist of air sacs (which are lined with a layer of thin- walled cells and surrounded by very fine blood vessels) and air tubes (see Fig. 83). The filling of the air sacs with fresh air and the emptying of the lungs are brought about by the action of (i) mus- cles attached to the ribs and (2) a large muscular organ called the diaphragm (di'a fram). This separates the chest cavity from the abdominal cavity (Fig. 84). When the muscles of the ribs and of the diaphragm relax, the chest cavity shrinks; this forces the air out of the lungs. By contracting the muscles of the ribs and the diaphragm we force the chest cavity to enlarge, reducing the pressure in the lungs and drawing air into them. Inspiration and expiration, the two movements of air in respiration, are thus brought about by the alternate ex- pansion and contraction of the thoracic cavity. There are sev-. Fig. movements of breathing in man When the muscular parti- tion(called the diaphragm) between the chest cavity and the abdominal cavity is pulled down, the chest cavity is enlarged. When the ribs are raised, the chest cavity is also enlarged. The rib muscles and the dia- phragm normally work in unison, alternately expand- ing and contracting the chest cavity. The shaded portion of the diagram shows the expanded con- dition— ribs raised and diaphragm
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