348 THE RESPIRATORY SYSTEM 



onset of pneumonia. The skin areas and the muscles in 

 question are supplied by the lower intercostal nerves, which 

 are also stated to supply the costal pleura. It is, however, by 

 no means certain whether the condition represents true 

 viscero-sensory and viscero-motor reflexes, or whether the 

 intercostal nerves are directly stimulated by the inflamed 

 costal pleura, with which they are in contact for a short 

 distance before they pass between the external and the internal 

 intercostal muscles. 



When the diaphragmatic pleura is inflamed, the terminal 

 branches of the phrenic nerve are stimulated. The afferent 

 impulses, therefore, pass to the fourth cervical segment of the 

 spinal medulla, and they sometimes become conveyed to the 

 cells which receive their impulses from the sensory branches 

 of the fourth cervical nerve. When this occurs, pain is 

 experienced in the area of distribution of the supra-clavicular 

 nerves (Fig. 69). 



The Lungs. The Lungs occupy the pleural sacs. They 

 contain a large amount of elastic tissue, which causes them to 

 contract when the pleural sacs are opened. This elasticity 

 is not sufficient to expel all the air from the alveoli, and 

 therefore pieces of lung tissue float in water. In a foetus 

 which has not breathed the alveoli do not contain air, and 

 portions pf Upe lungs will sink when they are immersed in 

 water. 



The lungs are somewhat pyramidal in shape. The rounded 

 apex extends upwards and completely fills the cupula pleurae. 

 The base is hollowed out to adapt itself to the shape of the 

 diaphragm, on which it rests. The costal surface is separated 

 from the mediastinal surface by a thin anterior and a rounded 

 posterior border. 



The left lung is divided into two lobes, an upper and a lower, 

 by the oblique fissure, which cuts through the lung substance as 

 far as the hilus, so that there is little or no structural continuity 

 between the two lobes. On this account, disease cannot spread 

 directly from one lobe to the other, unless their opposed surfaces 



