430 THE NERVOUS REGULATION OF THE BLOOD-VESSELS 



fibers modify the rate and force of the heart in such a degree that 

 it becomes quite impossible to recognize pure vasomotor changes. 

 For this reason, much stress cannot be placed upon the experiments 

 of Parker^ and Maas^ who measured the outflow from the coronary 

 veins of isolated hearts of cats while these organs were being perfused 

 through their coronary arteries. Under these conditions, the excitation 

 of the vagus led to a diminution and the stimulation of the sympathetic 

 fibers to an increase in the flow. In accordance with the foregoing 

 statement, we are not justified in attributing the former effect to a 

 vasoconstriction and the latter to a vasodilatation. 



For the same reason no definite conclusions can be drawn from 

 the observations of N. Martin,^ showing that the stimulation of the 

 vagus produces an enlargement of the smaller blood-vessels situated 

 in the surface layers of the heart and that a dilatation of these channels 

 results early during the state of asphyicia, when the general blood 

 pressure preserves as yet a perfectly normal value. Schafer,^ as well as 

 Wiggers,^ is of the opinion that the changes following the stimulation 

 of the cardiac nerves during perfusion may be explained more satis- 

 factorily by attributing them to other than vasomotor influences. 

 It has been reported, however, that the coronary vessels of the 

 quiescent heart constrict in response to adrenalin, and that this agent 

 increases the flow through this organ by modifying the character of 

 its contractions. 



B. THE PULMONARY CIRCULATION^ 



The dynamical factors which are responsible for the flow of the blood 

 through the lesser circuit, present the same general characteristics as 

 those previously discussed in connection with the greater circuit. The 

 pressure in the pulmonary artery finds its origin in the activity of the 

 right ventricle. As the driving force developed by this chamber 

 is relatively sHght, it cannot surprise us to find that the entire pul- 

 monary circulation is carried on with the aid of a rather low pressure 

 and, hence, with a lesser expenditure of energy, than the systemic. 

 But this statement is not meant to imply that the pulmonary circula- 

 tion is less effective, but merely to suggest that the low pressures here 

 pre trailing, are made possible by the fact that the resistance in this 

 circuit is very slight. That this deduction is correct may be gathered 

 from the observation that the pulmonary arterioles possess a larger 

 caliber and are equipped with only a scanty amount of smooth muscle 

 tissue. 



The blood-vessels of the lungs are constantly undergoing passive 



1 Boston Med. and Surg. Jour., 1896. 



2Pfluger's Archiv, Ixxiv, 1899, 281; also see: Dogiel and Archangelski, ibid., 

 cxvi, 1907, 482. 



3 Transact., Med. and Chir. Fac. of Maryland, 1891. 



^ Arch, des sciences biol., xi, Suppl., 1899. 



6 Am. Jour, of Physiol., xxiv, 1909, 391. 



s Discovered by Servet and Columbo during the middle of the 16th century. 



