470 



THE RESPIRATORY MECHANISM. 



fibers from the second, third, and fourth thoracic ganglia, fol- 

 lows the bronchi to their ultimate subdivisions. We have em- 

 phasized the fact that in the heart the vagal fibers served to 

 regulate the pace of cardiac contractions, and that, while feeble 

 galvanization or increased adrenal activity, by stimulating the 

 oxidation processes of the vagal bulbar centers, quickened the 

 heart-beats, powerful stimulation arrested them. The same 

 phenomena are obtained under similar conditions in the case 

 of the lungs. As is well known, section of the vagi in the 

 neck causes the respirations to fall to four or five a minute; 

 feeble galvanization accelerates their number, while a suffi- 

 ciently strong current will arrest the respiratory movements. 

 Barring the special functional attributes of the heart, which in 

 certain particulars influence experimental results, all phe- 

 nomena strictly ascribable to the vagal supply are repeated in 

 the lungs: a fact which pointedly attests to the similarity of 

 the nervous supplies of the two organs and to a common origin. 

 Still, we must not lose sight of the fact that the peripheral 

 respiratory muscles are the main factors in these phenomena, 

 and that those with which the lungs per se are concerned have 

 other objects in view. 



In the light of all we have said so far concerning the 

 active functional role assumed by the vasomotor terminal 

 branches of general motor nerves, the distribution of vaso- 

 motor fibers to the terminal arterioles must play a predomi- 

 nant part in pulmonary functions: one, indeed, of consider- 

 able clinical importance. Thus, the plexus which overlies the 

 pulmonary artery and is intermixed with vagal fibers becomes, 

 when considered from our standpoint, the regulative system 

 as regards the amount of venous blood allowed to penetrate 

 the lung. As this blood contains the adrenal secretion, this 

 vessel becomes, therefore, one of the most important pathways 

 of the general circulation. This asserts itself when we realize 

 that it is upon the amount of venous blood driven into the 

 lungs by the right ventricle, besides the character of this blood, 

 that the efficiency of the respiratory functions depends. Ex- 

 cessive activity of the adrenals increases the power of the 

 heart and vascular tension, the speed of the stream which 

 traverses the lungs is correspondingly augmented, and con- 



