344 PHYSIOLOGY. 



The Pleura. Each lung is enveloped by a serous membrane 

 the pleura composed of two layers, one of which is closely adherent 

 to the external surface of the lung; the other adheres to the inner 

 surface of the chest-wall. These layers are designated visceral and 

 parietal. The visceral pleura envelops the lung, while the parietal 

 pleura lines the thoracic wall. The two become continuous with one 

 another at the root of the lung. 



By this means two large serous sacs are formed, each distinct 

 and separate from the other. The pleural tissue is composed of a 

 layer of fibrous tissue covered with endothelium. During health the 

 two layers of the pleura are always in contact with one another, just 

 enough fluid being present between them to allow of their gliding 

 over one another with but very little friction during the accomplish- 

 ment of the respiratory acts. 



Lymphatics. These are very numerous in lung-tissue and so 

 arranged as to form several systems. 



Nerves. The nervous supply of the lungs is from the anterior 

 and posterior pulmonary plexuses derived from the vagus and sympa- 

 thetic. The nerves enter the lungs to folloAV the course of the 

 bronchi and their branches and end in the unstriped muscle. 



The function of the nonstriped muscular tissue of the lungs 

 seems to be to offer a general resistance to increased pressure within 

 the air-passages as may occur during forced expiration, as speaking, 

 singing, blowing, etc. The vagus is the nerve which supplies motor 

 fibers to these muscle-fibers. 



MECHANISM OF RESPIRATION. 



If respiration be suspended but a very short time there 

 will soon be felt a lively anxiety due to the nonsatisfaction of an 

 imperative need. This sensation of anxiety is produced by an inter- 

 nal sensation calling for need of breathing, it being promptly relieved 

 by the proper introduction of air into the lungs. When the air 

 inspired and retained becomes unfit for further oxidation, there 

 arises another internal sensation which calls for the expulsion of 

 that same air. Each respiratory movement is, therefore, preceded 

 by a particular sensation which commands its execution. 



These two movements constitute, by their regular succession, a 

 complete respiration, the purpose of which is to maintain in the lungs 

 regular currents which serve incessantly to renew the air altered by 

 its contact with the blood. The mechanism for the accomplishment 

 of respiration consists in an alternate dilatation and contraction of 



