658 THE CIRCULATION IN ASPHYXIA. [BOOK n. 



in an animal whose breathing is fairly normal. We need not 

 discuss them any further now, and have introduced them chiefly to 

 illustrate the fact that the vaso-motor nervous system is apt to fall 

 into a condition of rhythmic activity. 



389. While changes occurring primarily in the respiratory 

 system thus affect the vascular system, conversely changes occurring 

 primarily in the vascular system affect the respiratory system. 

 Two kinds of change in the vascular system bearing on two parts 

 of the respiratory system deserve especial attention. 



In the first place the respiratory mechanism may be affected 

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

 spinal bulb. We have already seen ( 371) that the sudden 

 cutting off of the supply of blood to the bulb gives rise to 

 dyspnceic 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 modified not 

 only by the general events but also by local events determining 

 its blood supply; it may for instance 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 bulb. 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 spinal bulb, 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 spinal bulb as well as the other tissues surfer 

 in consequence from a deficiency of oxygen. The deficient supply 

 to the bulb manifests itself in dyspnoeic or at least in laboured 

 breathing, which sometimes through the mechanical influences 



