478 MATERIA MEDIC A AND THERAPEUTICS. 



CHAPTER XI. 



THE RESPIRATOKY SYSTEM. 



I. PHYSIOLOGICAL RELATIONS. 



THE red corpuscle of the "blood is the oxygenating or 

 respiratory element of the "body. The physical part of 

 respiration is carried on by means of the chest and respiratory 

 passages, a fresh supply of oxygen being continually presented 

 to the red corpuscles, and carbonic acid, water, and heat given 

 off from the plasma. 



The red corpuscle and the chest are brought into functional 

 relation with each other by means of a special nervous 

 mechanism, called the respiratory centre, a portion of nervous 

 matter in the medulla oblongata which is peculiarly irritable in 

 the presence of oxygen, and sends motor impulses through 

 the cord to the respiratory muscles. 



The less the amount of oxygen admitted to the respiratory 

 centre, the more powerfully is it stimulated, and the chest 

 moved ; the greater the amount of oxygen admitted to the 

 centre, the less powerful its discharges, and the more weak or 

 superficial is the breathing. Now the amount of oxygen in the 

 arteries of the medulla is the same as in the systemic arteries 

 generally ; and we thus find that the state of oxygenation 

 of the arterial blood governs the respiratory movements through 

 the medium of the respiratory centre. The fundamental 

 canon in the physiology of respiration is that the condition of 

 the red corpuscle is the prime mover of all respiratory acts. Car- 

 bonic acid has no direct effect on the respiratory centre. 



The term " centre" implies that certain influences meet in 

 this point, originating in a circle of which it is the middle 

 point ; and this is the case. Falling into the respiratory centre 

 are impressions conveyed by afferent usually sensory nerves, 

 from every part of the body, modifying its activity, and 

 reflexly influencing the respiratory movements. The vagus 

 is peculiarly capable of stimulating the centre ; thus irritation 

 of the larynx immediately causes the reflex respiratory act 

 called cough. The vagus is therefore said to be the special 

 afferent nerve of respiration ; the whole surface of the 

 respiratory passages, and probably the lungs, being abundantly 

 supplied with rootlets of the vagus, which are incessantly 

 collecting impressions for transmission to the centre. Every 

 change in the distension of the lungs, and in the quantity and 

 quality of the pulmonary blood, thus instantly tells on the 



