228 RELATIONS OF THE VASCULAR FACTORS. [BOOK i. 



organs of the abdomen. Conversely, if peripheral resistance be in 

 any area increased, the general blood-pressure is prevented from 

 rising too high, by reason of the actual increase of blood-pressure 

 so affecting the medulla, that inhibitory impulses descend the 

 vagus, and, by producing a less frequent, possibly a weaker pulse, 

 tone down the distension of the arteries. 



The more we learn of the working of the body, the more aware 

 we become of the fact that it is crowded with regulative and 

 compensating arrangements no less striking and exquisite than the 

 two we have just described. Some of these will be seen in the 

 following almost tabular statement of the various modifications of 

 the vascular factors, and of their causes. 



A. The Beat of the Heart is affected 



1. By the amount of distension of the ventricular cavities pre- 

 ceding the systole. This will depend on 



a. The quantity of blood reaching the heart and passing into 

 its cavities during the diastole. This in turn is determined by 

 the flow of blood through the veins, the flow itself being influenced 

 by the arterial pressure, respiratory movements, &c. &c. 



b. The force of the auricular contractions. 



c. The amount of resistance which has to be overcome by the 

 systole. This is determined by the mean arterial pressure, and is 

 influenced by everything which influences that. 



2. By the quantity of the blood passing through the coronary 

 arteries. In the frog the thin walls of the auricle and the spongy 

 texture of the ventricle permit the nourishment of the cardiac sub- 

 stance to be carried on by direct contact with the blood in the 

 cavities. In mammals this mode of nutrition must be insignificant. 

 In them the condition of the cardiac muscles and nervous ap- 

 pendages depends almost exclusively on the blood distributed by 

 the coronary arteries. The coronary circulation however is peculiar 

 and is largely determined by the action of the heart itself. 



3. By the quality of the blood passing through the coronary 

 arteries, and acting upon simply the muscular tissue, or upon the 

 various nervous mechanisms, or upon both. This is illustrated by 

 the action of poisons. The quantitative relations of the normal, 

 and the presence of abnormal, constituents of the blood must of 

 necessity profoundly affect the heart's beat. 



4. Through the inhibitory fibres of the vagus. 



a. By the blood directly stimulating the endings of the vagus 

 fibres. This is only seen in the case of poisons. 



b. By the blood directly affecting the cardio-mhibitory centre 



