THE CIRCULATION OF THE BLOOD. 245 



block be complete, that portion of the heart wall supplied by the branch 

 dies. The immediate effect of the closure of a large branch in the dog 

 may be occasional and transient irregularity, or arrest of the ventricular 

 contractions, preceded by irregularities in the force of the contractions 

 and a diminution in the amount of work performed. The force, rather 

 than the rate, of the ventricular contractions is closely dependent upon 

 the blood-supply to the coronary arteries. 



(b.) The Peripheral Resistance. The regulation of the amount 

 of resistance to the passage of blood at the periphery is principally done 

 by the alteration of the calibre of the arterioles. This regulating power 

 is chiefly invested in the nervous system. Its influence is exerted upon 

 the muscular coat of the arteries and not upon the elastic element, which 

 possesses, as must be obvious, rather physical" than vital properties. 

 The muscular tissue in the walls of the vessels increases in amount rel- 

 atively to the other coats as the arteries grow smaller, so that in the 

 arterioles it is developed out' of all proportion to the other elements; 

 in fact, in passing from cap'illary vessels, made up as we have seen of 

 endothelial cells with a ground substance, the first change which occurs 

 as the vessels become larger (on the side of the arteries) is the appear- 

 ance of muscular fibres. Thus the nervous system is more poAverful in 

 regulating the calibre of the smaller than of the larger arteries. 



It was long ago shown by Claude Bernard that if the cervical sym- 

 pathetic nerve is divided in a rabbit, the blood-vessels of the correspond- 

 ing side of the head and neck become dilated. This effect is best seen 

 in the ear, which if held up to the light is seen to become redder, and 

 the arteries are seen to become larger. The whole ear is distinctly 

 warmer than the opposite one. This effect is produced by removing 

 the arteries from the influence of the central nervous system, which in- 

 fluence normally passes down the divided nerve; for if the peripheral 

 end of the divided nerve (i.e., that farthest from the brain) be stimulated, 

 the arteries which were before dilated return to their natural size, and 

 the parts regain their primitive condition. And, besides this, if the 

 stimulus is very strong or very long continued, the point of normal con- 

 striction is passed, and the vessels become much more contracted than nor- 

 mal. The natural condition, which is about midway between extreme con- 

 traction and extreme dilatation, is called the natural tone of an artery; 

 if this is not maintained, the vessel is said to have lost tone, or if it is 

 exaggerated, the tone is said to be too great. The effects described as 

 having been produced by section of the cervical sympathetic and by 

 subsequent stimulation are not peculiar to that nerve, as it has been 

 found that for every part of the body there exists a nerve the division 

 of which produces the same effects, viz., dilatation of the vessels; such 

 may be cited as the case with the sciatic, the splanchnic nerves,, and the 



