242 ANATOMY FOR NURSES [Chap. XIII 



of the pneumogastric. These nerves follow the course of the 

 blood-vessels and bronchioles. 



Pleura. — Each lung is enclosed in a serous sac, the pleura, 

 one layer of which is closely adherent to the walls of the chest 

 and dia])hragni (parietal) ; the other closely covers the lung 

 (visceral). The two layers of the pleural sacs, moistened by 

 serum, are normally in close contact ; they move easily upon 

 one another, and prevent the friction that would otherwise occur 

 between the lungs and the walls of the chest with every respiration. 

 Inflammation of the pleura is called pleurisy. 



Mediastinum. — The mediastinum is the space between the two 

 pleural sacs. It extends from the sternum to the spinal column 

 and holds a portion of many organs, i.e. the trachea, oesophagus, 

 great vessels connected with the heart, ner^'es, and the thoracic 

 duct. 



RESPIRATION 



Function. — The main purpose of respiration is to supply the body 

 with oxygen and get rid of the excess carbon dioxide which results 

 from oxidation. It also helps to equalize the temperature of the 

 body and get rid of excess of water. To accomplish these purjioses 

 two processes are necessary, i.e. external and internal respiration. 



External respiration. — This takes place in the lungs and con- 

 sists of the absorption of oxygen from the air in the alveoli, and the 

 elimination of some of the carbon dioxide and water from the 

 blood in the capillaries. External respiration consists of inspira- 

 tion, or the process of taking air into the lungs, and expiration, or 

 the process of expelling air from the lungs. 



Internal respiration. — This takes place in the cells and consists 

 of the diffusion of oxygen from the blood in the capillaries into the 

 tissues, and its union there with the protoplasm of the cells. As 

 a result of this union or oxidation complex bodies are broken 

 up into simpler ones, such as carbon dioxide and water, and 

 there is thus liberated a great deal of energy, which is mani- 

 fested in the increasing of muscular activity and in the pro- 

 duction of heat. The carbon dioxide passes by diffusion into 

 the venous blood, and is carried by it to the right side of the 

 heart and thence to the lungs, a certain quantity, however, 

 escaping from the blood through the kidneys and skin. 



