CHAP, ii.] RESPIRATION. 627 



by changes in the blood supply to the respiratory centre in the 

 medulla. We have already seen ( 371), that the sudden cutting 

 off of the supply of blood to the medulla gives rise to dyspnoeic 

 respiratory movements and may lead to expiratory convulsions. 

 That is an extreme case; but short of that, the activity of the 

 respiratory centre, the extent and character of the respiratory 

 explosions which take place in it, may be varied according as the 

 constricted or dilated condition of the small arteries branching off 

 from the basilar artery or of the basilar artery itself allows a scanty 

 or a full flow of blood through the medulla. And it is possible 

 that some forms of dyspnoea may be brought about in this way. 



Much more common and important however is the second kind 

 of change, that affecting the circulation through the lungs. In 

 the normal organism an .adequate supply of arterial blood to the 

 tissues is secured by an adequate renewal of the air in the 

 pulmonary alveoli and an adequately rapid flow of blood through 

 the pulmonary capillaries. When, as by obstruction in the pulmo- 

 nary arteries, or by failure of the cardiac valves, or, and perhaps 

 especially, by an insufficient cardiac stroke, the stream of blood from 

 the lungs into the left ventricle is lessened either in amount or in 

 rapidity, less oxygen is carried to the tissues, including the nervous 

 tissue of the medulla, and dyspnoea or " want of breath " follows. 

 When the circulation through the lungs is in full healthy swing, 

 the haemoglobin of the red corpuscles is as we have seen saturated 

 or nearly saturated with oxygen. If owing to a slower stream the 

 red corpuscles tarry longer in their passage along the walls of the 

 pulmonary alveoli they cannot thereby take up a compensating 

 addition of oxygen, indeed it is doubtful if they can take up any 

 additional oxygen at all. The blood falling under these circum- 

 stances into the left ventricle and sent thence over the body is 

 not more arterial than usual ; at the same time the amount of 

 blood sent out at each heart stroke is less, often much less, than 

 the normal ; and the medulla as well as the other tissues suffer in 

 consequence from a deficiency of oxygen. The deficient supply to 

 the medulla manifests itself in dyspnoeic or at least in laboured 

 breathing, which sometimes through the mechanical influences 

 discussed above has the happy result of improving the pulmonary 

 circulation and so produces compensating effects. When the 

 pulmonary artery is suddenly plugged with a clot the primary and 

 urgent symptom is " want of breath," though air enters freely into 

 the chest ; and " cardiac dyspnoea " is a common symptom of 

 cardiac disease. 



390. Other systems of the body are also related to the 

 respiratory system, though by ties less striking than those which 

 bind to it the vascular system. We have seen that deficient 

 arterialisation of the blood stirs up the muscles of the alimentary 

 canal to increased activity, and we shall presently see that the 

 same condition has a notable effect in promoting the perspiration ; 



402 



