RESPIRATION. 177 



Action of the nervous system on circulation. 



The heart is under control of both the cerebro-spinal and sym- 

 pathetic nervous systems, being supplied by the pneumogastric 

 nerve and the cervical sympathetic, there being also probably 

 some intracardiac sympathetic ganglia. 



The pneumogastric has an inhibitory action on the heart, as 

 shown by section of the nerves, which is followed by an increased 

 frequency of the heart's action. 



The sympathetic has an accelerative action on the heart, as 

 shown by the previous experiment, and also by stimulation of the 

 fibres, which is followed by an increased number of pulsations. 

 Stimulation of the tenth nerve is followed by decreased pulsations, 

 and if a strong current is used, by paralysis of the heart in dias- 

 tole, by stimulation of its inhibitory action. 



Local circulation is regulated by the vaso-motor nerves of the 

 sympathetic system. They are known as vaso-dilators and vaso- 

 contr actors. 



RESPIRATION. 



Respiration consists of an appropriation of oxygen by the tissues 

 and a giving off of carbonic oxide gas. 



The respiratory organs consist, from above downward, of the 

 (i) nasal fossa, (2) pharynx, (3) larynx, (4) trachea, (5) bronchi, 

 and (6) lungs. (For anatomical descriptions, see Viscera.) 



The different steps of respiration consist 



i st. Of the introduction of atmospheric air into the lungs in in- 

 spiration. 



2d. Of the taking up of oxygen by the blood. 



3d. Of the appropriation of oxygen by the tissues. 



4th. Of the exhalation of carbonic acid gas in expiration. 



In Inspiration the air passes through the mouth or nasal cavi- 

 ties to the larynx, the glottis being dilated, then to the alveoli 

 of the lungs, which it distends. 



Muscles of Inspiration. 



The diaphragm is the most important, becoming flattened, 

 and at the convexity decreased. 



The external intercostals and supercostals raise the ribs and 

 carry them forward, thus enlarging the lateral diameter of the 

 thorax. 



